Add a Cheats menu tab with engine cheat buttons and unlimited-resource switches

The new Cheats tab exposes the engine's own cheat handlers as buttons:
give all items and weapons, toggle physics, next level and previous
level. Physics shows its state through the button highlight. The buttons
need a running game and show a message otherwise.

It also has four independent session-only switches for unlimited health,
energy, ammo and items (grenades and drugs). While a switch is on, its
resource is topped up every frame by qs_cheat_refill() from the new
engine patch 26. The switches are not saved to the config.

CONFIG.md

@@ -358,7 +358,7 @@ keyboard (there is no hold behavior).
 - `keypad 0`-`keypad 9`: a controller button per digit is not useful; use the
   on-screen keyboard.
 - `toggle_freelook` (F): the free-view actions cover mouse look.
-- The 4 `cheat_*` entries: debug cheats.
+- The 4 `cheat_*` entries: debug cheats. They are on the menu's Cheats tab instead.
 - `mac_help` (disabled) and `audiolog_cancel` (its build flag is not
   defined): they are not real hotkeys in this build.
 - `toggle_opengl` (Ctrl+G): the Video Options tab's Renderer buttons already