qt: Rework ui pause update into a slot for thread safety

src/qt/qt_mainwindow.hpp

@@ -33,7 +33,6 @@ public:
     void blitToWidget(int x, int y, int w, int h, int monitor_index);
     QSize getRenderWidgetSize();
     void setSendKeyboardInput(bool enabled);
-    void setUiPauseState(bool paused);
 
     std::array<std::unique_ptr<RendererStack>, 8> renderers;
 signals:
@@ -65,6 +64,7 @@ public slots:
     void togglePause();
     void initRendererMonitorSlot(int monitor_index);
     void destroyRendererMonitorSlot(int monitor_index);
+    void updateUiPauseState();
 private slots:
     void on_actionFullscreen_triggered();
     void on_actionSettings_triggered();