Add SDL_IsTraySupported

[Semphriss]

Description

Allows detecting whether or not trays should be supported, without having to create a tray icon.

I added a note that it's not a guarantee that it will/won't work when calling SDL_CreateTray, and that the availability may vary with time.

Existing Issue(s)

Fixes #13119

[icculus]

I'd probably take out the documentation comment about how this can change during the course of the program if system libraries change...I think that's beyond our scope.

In fact, I'd just cache the result after the first check, on platforms where it isn't universally true.

[Semphriss]

I updated the code; is it fine if I do the caching stuff in the platform implementations, or should I add something like SDL_SYS_IsTraySupported and use a global function to do the caching for all platforms?

[icculus]

should I add something like SDL_SYS_IsTraySupported

I would prefer this because it matches what we do in other subsystems (and I would move the check for the main thread there, too, even if a backend doesn't strictly require it, so behavior is consistent across platforms), but that's a task for the future; this PR can land as-is.