Add support for modifier only shortcuts on Wayland

On popular demand!

This change tracks how modifiers are used and detects a modifier only
key press/release. That is:
* no other key is pressed when the modifier gets pressed
* no other key gets pressed before the modifier gets released

If such a press/release is detected, we call a configurable dbus call.
The possible shortcuts can be configured in kwinrc, group
"ModifierOnlyShortcuts". The following keys are supported:
* Shift
* Control
* Alt
* Meta

As value it takes a QStringList (comma seperated string) with
service,path,interface,method,additionalargs

E.g. to invoke Desktop Grid effect on Meta key:

[ModifierOnlyShortcuts]
Meta=org.kde.kglobalaccel,/component/kwin/,org.kde.kglobalaccel.Component,invokeShortcut,ShowDesktopGrid

I do not intend to add a config interface for it. Let's keep it a hidden
way.

REVIEW: 124954

input.cpp

@@ -42,6 +42,8 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #include <decorations/decoratedclient.h>
 #include <KDecoration2/Decoration>
 // Qt
+#include <QDBusMessage>
+#include <QDBusPendingCall>
 #include <QKeyEvent>
 #include <QMouseEvent>
 #include <QTemporaryFile>
@@ -183,6 +185,32 @@ void Xkb::updateKey(uint32_t key, InputRedirection::KeyboardKeyState state)
     }
     xkb_state_update_key(m_state, key + 8, static_cast<xkb_key_direction>(state));
     updateModifiers();
+    if (state == InputRedirection::KeyboardKeyPressed) {
+        m_modOnlyShortcut.pressCount++;
+        if (m_modOnlyShortcut.pressCount == 1) {
+            m_modOnlyShortcut.modifier = Qt::KeyboardModifier(int(m_modifiers));
+        } else {
+            m_modOnlyShortcut.modifier = Qt::NoModifier;
+        }
+    } else {
+        m_modOnlyShortcut.pressCount--;
+        // TODO: ignore on lock screen
+        if (m_modOnlyShortcut.pressCount == 0) {
+            if (m_modOnlyShortcut.modifier != Qt::NoModifier) {
+                const auto list = options->modifierOnlyDBusShortcut(m_modOnlyShortcut.modifier);
+                if (list.size() >= 4) {
+                    auto call = QDBusMessage::createMethodCall(list.at(0), list.at(1), list.at(2), list.at(3));
+                    QVariantList args;
+                    for (int i = 4; i < list.size(); ++i) {
+                        args << list.at(i);
+                    }
+                    call.setArguments(args);
+                    QDBusConnection::sessionBus().asyncCall(call);
+                }
+            }
+        }
+        m_modOnlyShortcut.modifier = Qt::NoModifier;
+    }
 }
 
 void Xkb::updateModifiers()

input.h

@@ -262,6 +262,10 @@ private:
     xkb_mod_index_t m_altModifier;
     xkb_mod_index_t m_metaModifier;
     Qt::KeyboardModifiers m_modifiers;
+    struct {
+        uint pressCount = 0;
+        Qt::KeyboardModifier modifier = Qt::NoModifier;
+    } m_modOnlyShortcut;
 };
 
 inline

options.cpp

@@ -838,6 +838,22 @@ void Options::loadConfig()
     setMaxFpsInterval(1 * 1000 * 1000 * 1000 / config.readEntry("MaxFPS", Options::defaultMaxFps()));
     setRefreshRate(config.readEntry("RefreshRate", Options::defaultRefreshRate()));
     setVBlankTime(config.readEntry("VBlankTime", Options::defaultVBlankTime()) * 1000); // config in micro, value in nano resolution
+
+    // Modifier Only Shortcuts
+    config = KConfigGroup(m_settings->config(), "ModifierOnlyShortcuts");
+    m_modifierOnlyShortcuts.clear();
+    if (config.hasKey("Shift")) {
+        m_modifierOnlyShortcuts.insert(Qt::ShiftModifier, config.readEntry("Shift", QStringList()));
+    }
+    if (config.hasKey("Control")) {
+        m_modifierOnlyShortcuts.insert(Qt::ControlModifier, config.readEntry("Control", QStringList()));
+    }
+    if (config.hasKey("Alt")) {
+        m_modifierOnlyShortcuts.insert(Qt::AltModifier, config.readEntry("Alt", QStringList()));
+    }
+    if (config.hasKey("Meta")) {
+        m_modifierOnlyShortcuts.insert(Qt::MetaModifier, config.readEntry("Meta", QStringList()));
+    }
 }
 
 void Options::syncFromKcfgc()
@@ -1124,4 +1140,9 @@ Options::WindowOperation Options::operationMaxButtonClick(Qt::MouseButtons butto
            opMaxButtonLeftClick;
 }
 
+QStringList Options::modifierOnlyDBusShortcut(Qt::KeyboardModifier mod) const
+{
+    return m_modifierOnlyShortcuts.value(mod);
+}
+
 } // namespace

options.h

@@ -589,6 +589,8 @@ public:
         return m_glPreferBufferSwap;
     }
 
+    QStringList modifierOnlyDBusShortcut(Qt::KeyboardModifier mod) const;
+
     // setters
     void setFocusPolicy(FocusPolicy focusPolicy);
     void setNextFocusPrefersMouse(bool nextFocusPrefersMouse);
@@ -929,6 +931,8 @@ private:
     bool condensed_title;
     int animationSpeed; // 0 - instant, 5 - very slow
 
+    QHash<Qt::KeyboardModifier, QStringList> m_modifierOnlyShortcuts;
+
     MouseCommand wheelToMouseCommand(MouseWheelCommand com, int delta) const;
 };