[Feature Request] Engine-side pause menu

[ArcticFqx]

Currently the pause screen is defined in Lua in a bunch of ways theme-side.

Ideally, the pausing of the game should be done engine-side as a screen overlayed on top of the screen stack on ScreenGameplay and friends. A suggested name for this new screen is ScreenPauseMenu.

The screen should have a resume, restart and forfeit song button.
Themes should be able to add other buttons or features to this screen.

To retain compatibility on older themes which roll their own menus, maybe a metric should be used to toggle this feature on?