break showingDesktop w/ tabbox/PW/DG

This is now crucial, because while before
(the minimized) windows were conditionally
shown, but are now always behind the desktop.
Also, it makes the tabbox more consistent.

BUG: 344083
REVIEW: 122679

effects/desktopgrid/desktopgrid.cpp

@@ -1039,9 +1039,11 @@ void DesktopGridEffect::setActive(bool active)
         return; // Already in that state
 
     activated = active;
-    if (activated && timeline.currentValue() == 0)
-        setup();
-    if (!activated) {
+    if (activated) {
+        effects->setShowingDesktop(false);
+        if (timeline.currentValue() == 0)
+            setup();
+    } else {
         if (isUsingPresentWindows()) {
             QList<WindowMotionManager>::iterator it;
             for (it = m_managers.begin(); it != m_managers.end(); ++it) {

effects/presentwindows/presentwindows.cpp

@@ -1422,6 +1422,7 @@ void PresentWindowsEffect::setActive(bool active)
         return;
     m_activated = active;
     if (m_activated) {
+        effects->setShowingDesktop(false);
         m_needInitialSelection = true;
         m_closeButtonCorner = (Qt::Corner)effects->kwinOption(KWin::CloseButtonCorner).toInt();
         m_decalOpacity = 0.0;

tabbox/tabbox.cpp

@@ -730,6 +730,7 @@ void TabBox::show()
         m_isShown = false;
         return;
     }
+    workspace()->setShowingDesktop(false);
     reference();
     m_isShown = true;
     m_tabBox->show();