Generalized multi-instance

[hrydgard]

Takes #13169 and extracts it to a separate file, and changes how it's called.

Instance counter is now initialized on startup, enabling us to do the following things for secondary instances:

• They don't save their configs
• They are silent (can be overridden at runtime if desired).
• The title bar shows the instance number (Windows only so far).

Which makes the experience a lot better.

Possible alternatives would be to have a separate configs: ppsspp2.ini, ppsspp3.ini and so on, but felt this was simpler.

[LunaMoo]

Since PPSSPP will now oficially support multiply instances on same pc, it's worth noting that some people were requesting to disable(on windows) titlebar, borders and win menu(as they play like that with a single display).

I made options for that long ago:
LunaMoo@3c49485
LunaMoo@9b0b239
LunaMoo@06e90a7
could open a PR with those options, through not sure which menu should it go, it might also need some more "automatic" function for placing windows(since they can't be moved around with titlebar disabled) considering the new way doesn't save configs. IE it could just place each instance window in different corner up to 4 or something.

[hrydgard]

I can see how those options would be useful. I think we could make things even more convenient with some sort of auto-tile that just does all that in fullscreen mode automatically if there are multiple instances around - then you'd need no options, you'd just go into fullscreen and it would automatically tile the two instances borderlessly. And the windows would be restored coming out of full screen. Would just need a little window message trickery ...

[LeoKids]

This might help me getting problems using Netplay on Naruto Ultimate Ninja Heroes.