UI: Massive revamp, new features and improvements

src/frontend-common/imgui_overlays.h

@@ -0,0 +1,22 @@
+#pragma once
+
+#include "imgui_manager.h"
+
+namespace ImGuiManager {
+void RenderOverlays();
+}
+
+namespace SaveStateSelectorUI {
+
+static constexpr float DEFAULT_OPEN_TIME = 5.0f;
+
+void Open(float open_time = DEFAULT_OPEN_TIME);
+void Close();
+
+void SelectNextSlot();
+void SelectPreviousSlot();
+
+void LoadCurrentSlot();
+void SaveCurrentSlot();
+
+} // namespace SaveStateSelectorUI