[input]Add caps and numlock to the modifiers

[matham]

Fixes #6285.

I looked at https://wiki.libsdl.org/SDL_Keymod and SDL_keycode.h and updated the code based on this.

Specifically,

• I renamed the KMOD_xMETA key to KMOD_xGUI, because that's what sdl2 calls it (it's the key with the windows icon typically).
• I added KMOD_MODE, this is basically the same as the right alt key.
• I added the KMOD_NUM and KMOD_CAPS keys, which are the numslock and capslock keys.

[pythonic64]

@matham Should Scroll Lock also be added to modifiers?

[matham]

I didn't see it in the sdl2 list. I added most everything from their site.

But also, probably not as it is probably not used much? We can always add it later if someone does need it.

[pythonic64]

I agree that it is not probably used much, but it would make modifiers list complete imho.
This issue #5062 is still present in current master ca3a68d.
Can you take a look at its pull request? Just asking because you knowledge of key handling.

[matham]

I'm not really very familiar with the keyboard logic. Also, it'a not super clear to me how ready that PR is to be merged (for starters it has merge conflicts).