QT Frontend: Add a Loading screen with progressbar

With shader caches on the horizon, one requirement is to provide visible
feedback for the progress. The shader cache reportedly takes several
minutes to load for large caches that were invalidated, and as such we
should provide a loading screen with progress.

Adds a loading screen widget that will be shown until the first frame of
the game is swapped. This was chosen in case shader caches are not being
used, several games still take more than a few seconds to launch and
could benefit from a loading screen.

src/yuzu/bootmanager.h

@@ -152,6 +152,7 @@ public slots:
 signals:
     /// Emitted when the window is closed
     void Closed();
+    void FirstFrameDisplayed();
 
 private:
     std::pair<unsigned, unsigned> ScaleTouch(const QPointF pos) const;
@@ -171,6 +172,8 @@ private:
     /// Temporary storage of the screenshot taken
     QImage screenshot_image;
 
+    bool first_frame = false;
+
 protected:
     void showEvent(QShowEvent* event) override;
 };