Preliminary support for saving to an arbitrary saved state slot.

Currently, tapping a cell representing a saved state slot in the menu will load it, regardless of the current option, save or load. So the save option only accepts Left, Right and A.
Make "Create saved state" have its own row of slot cells.

Raise the number of cells to 16.

Cache existence of saved state files, and invalidate the cache when saving or deleting a saved state.

CATSFC/system/gui: Make the unselected and selected cells more visibly different, while also leaving full and empty cells visibly different.
The delete (delette?) save state submenu still had 10 slots hardcoded.
Make yes/no dialog(ue)s respond to touches on the Yes and No buttons.
Remove some comments in draw_yesno_dialog to make it easier to read.
Raise the CPU frequency while loading or saving states and while showing their screenshots.
Forgot to define inputdata for the button touch commit.
Release 1.19.