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/CMakeLists.txt

@@ -115,6 +115,9 @@ add_executable(citra-qt
     game_list_worker.h
     hotkeys.cpp
     hotkeys.h
+    loading_screen.cpp
+    loading_screen.h
+    loading_screen.ui
     main.cpp
     main.h
     main.ui