Unify background colour override settings

Goals:

• Background colour becomes a frontend preference, and existing core settings for background colour override should be replaced with bools, with cores reading the user's chosen colour from the frontend somehow.
• Single hotkey for enabling/disabling override on all cores (see also #3091 as that should probably be designed first).
• When setting is disabled, core should use fully-transparent black if possible, or else solid black. (Does the frontend support transparency? We recently had that PSX screenshot issue and IIRC it was solved with an |= 0xFF in the frontend, so I'm thinking yes.)
• If a core isn't able to use an arbitrary colour for the background (ZXHawk), it chooses the closest available colour by whatever method.
• Video buffer on frame 0 (as captured in savestates) is filled with the user's background colour (only if the core has the setting and it's enabled?).

see also #1642

TODO add labels for other relevant cores

Start by reviewing the related issues #3091 and #1642, then trace the existing frontend and core background-colour settings. Done means the setting and hotkey are unified, supported cores handle the chosen or fallback colour, and the specified frame-0 video-buffer behavior is covered.