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/loading_screen.h

@@ -0,0 +1,78 @@
+// Copyright 2020 Citra Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <chrono>
+#include <memory>
+#include <QString>
+#include <QWidget>
+
+namespace Loader {
+class AppLoader;
+}
+
+namespace Ui {
+class LoadingScreen;
+}
+
+namespace VideoCore {
+enum class LoadCallbackStage;
+}
+
+class QGraphicsOpacityEffect;
+class QPropertyAnimation;
+
+class LoadingScreen : public QWidget {
+    Q_OBJECT
+
+public:
+    explicit LoadingScreen(QWidget* parent = nullptr);
+
+    ~LoadingScreen();
+
+    /// Call before showing the loading screen to load the widgets with the logo and banner for the
+    /// currently loaded application.
+    void Prepare(Loader::AppLoader& loader);
+
+    /// After the loading screen is hidden, the owner of this class can call this to clean up any
+    /// used resources such as the logo and banner.
+    void Clear();
+
+    /// Slot used to update the status of the progress bar
+    void OnLoadProgress(VideoCore::LoadCallbackStage stage, std::size_t value, std::size_t total);
+
+    /// Hides the LoadingScreen with a fade out effect
+    void OnLoadComplete();
+
+    // In order to use a custom widget with a stylesheet, you need to override the paintEvent
+    // See https://wiki.qt.io/How_to_Change_the_Background_Color_of_QWidget
+    void paintEvent(QPaintEvent* event) override;
+
+signals:
+    void LoadProgress(VideoCore::LoadCallbackStage stage, std::size_t value, std::size_t total);
+    /// Signals that this widget is completely hidden now and should be replaced with the other
+    /// widget
+    void Hidden();
+
+private:
+    std::unique_ptr<Ui::LoadingScreen> ui;
+    std::size_t previous_total = 0;
+    VideoCore::LoadCallbackStage previous_stage;
+
+    QGraphicsOpacityEffect* opacity_effect = nullptr;
+    std::unique_ptr<QPropertyAnimation> fadeout_animation;
+
+    // Variables used to keep track of the current ETA.
+    // If the rolling_average * shaders_remaining > eta_break_point then we want to display the eta.
+    // We don't want to always display it since showing an ETA leads people to think its taking
+    // longer that it is because ETAs are often wrong
+    static constexpr std::chrono::seconds ETABreakPoint = std::chrono::seconds{10};
+    static constexpr std::size_t NumberOfDataPoints = 25;
+    std::chrono::high_resolution_clock::time_point previous_time;
+    std::chrono::duration<double> rolling_average = {};
+    bool eta_shown = false;
+};
+
+Q_DECLARE_METATYPE(VideoCore::LoadCallbackStage);