Add transparent overlay window (#4746)

src/widgets/splits/Split.hpp

@@ -21,6 +21,7 @@ class SplitInput;
 class SplitContainer;
 class SplitOverlay;
 class SelectChannelDialog;
+class OverlayWindow;
 
 // Each ChatWidget consists of three sub-elements that handle their own part of
 // the chat widget: ChatWidgetHeader
@@ -80,6 +81,8 @@ public:
     // This is called on window focus lost
     void unpause();
 
+    OverlayWindow *overlayWindow();
+
     static pajlada::Signals::Signal<Qt::KeyboardModifiers>
         modifierStatusChanged;
     static Qt::KeyboardModifiers modifierStatus;
@@ -158,6 +161,8 @@ private:
     SplitInput *const input_;
     SplitOverlay *const overlay_;
 
+    QPointer<OverlayWindow> overlayWindow_;
+
     QPointer<SelectChannelDialog> selectChannelDialog_;
 
     pajlada::Signals::Connection channelIDChangedConnection_;
@@ -179,6 +184,7 @@ public slots:
     void explainMoving();
     void explainSplitting();
     void popup();
+    void showOverlayWindow();
     void clear();
     void openInBrowser();
     void openModViewInBrowser();