QT Frontend: Add disk shader loading progress bar

Until we get a on screen display or async shader loading, we should at
least have some measure of progress in the meantime. This is 90% a port
from the loading screen I made for yuzu, but with a slightly different
changed detection for when to display the ETA. Now we keep track of a
rolling estimate for shader load ETA and only display a ETA if its going
to take longer than 10 seconds.

src/citra_qt/main.h

@@ -32,6 +32,7 @@ class GraphicsVertexShaderWidget;
 class GRenderWindow;
 class IPCRecorderWidget;
 class LLEServiceModulesWidget;
+class LoadingScreen;
 class MicroProfileDialog;
 class MultiplayerState;
 class ProfilerWidget;
@@ -75,6 +76,7 @@ public:
 
 public slots:
     void OnAppFocusStateChanged(Qt::ApplicationState state);
+    void OnLoadComplete();
 
 signals:
 
@@ -221,6 +223,7 @@ private:
     GRenderWindow* render_window;
 
     GameListPlaceholder* game_list_placeholder;
+    LoadingScreen* loading_screen;
 
     // Status bar elements
     QProgressBar* progress_bar = nullptr;