Add skin cycling with shortcuts for next and previous skin

[ArijanJ]

This adds shortcuts for 'next skin' and 'previous skin', letting players switch skins more consistently than the 'random skin' option.

To implement this I had to I move the skin dropdown list generation (default skins + 'random skin' + user unprotected skins) into a separate function GetDropdownItems() to avoid duplication. The behavior stays the same from my testing with importing/deleting skins.

I added CycleSkins(int step), then two wrappers for it (SelectNextSkin and SelectPreviousSkin), which are called by the global shortcuts.

There was also a private SkinInfo object called random_skin_info that I removed in favor of comparisons with the public SkinInfo.RANDOM_SKIN ID (there was only one comparison to the object).

However, whenever the skin is changed for any reason, this tooltip shows up showing the random skin shortcut. I'll leave it, since cramming the two new tooltips into it would be ugly.

[ArijanJ]

Ok, this should address everything, only thing left is to find better shortcuts.

[peppy]

my best proposal if we do want default binds are probably E/T, as in left and right on keyboard from the "random skin" binding.

[ArijanJ]

Yep, neat idea. Ready to go unless you have any other notes.

[peppy]

@bdach would appreciate a five minute check on this since i refactored a good portion.

[bdach]

i dunno about the feature but the code seems solid enough i guess