Quick tile only resizeable clients

Summary:
Maximized clients weren't considered as resizeable when quick tiling was
added. Therefore, a special case was added in Client::setQuickTileMode().
However, that special case didn't take into account that a fullscreen
client can be also maximized. Clearly, we don't want users quick tile
fullscreen clients.

BUG: 411028
FIXED-IN: 5.17.0

Test Plan: No longer able to quick tile maximized fullscreen window of Konsole.

Reviewers: #kwin

Subscribers: romangg, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D23604

geometry.cpp

@@ -3221,9 +3221,10 @@ QRect AbstractClient::electricBorderMaximizeGeometry(QPoint pos, int desktop)
 
 void AbstractClient::setQuickTileMode(QuickTileMode mode, bool keyboard)
 {
-    // Only allow quick tile on a regular or maximized window
-    if (!isResizable() && maximizeMode() != MaximizeFull)
+    // Only allow quick tile on a regular window.
+    if (!isResizable()) {
         return;
+    }
 
     workspace()->updateFocusMousePosition(Cursor::pos()); // may cause leave event