front: Enabled virtual gamepad image sharing
An optional parameter "images" can now be specified in the "INFO" section.
When specified, button and mask images will be pulled from that folder
(allowing layouts to contain only a pad.ini file, and reducing space).
Old custom layouts are still backward compatible. Additionally, I added an
all buttons layout.  "Small" and "Tiny" layouts do not currently work
(These will be replaced by image scaling in the next commit).