Extra themability features
Two enhancements to the ability to reskin the Wimp:
- Switches to set the button border and face colours (per Ticket #378) to better match the rest of the theme, eg. less harsh on a pastel background
- Look for an optional 'tile_i' sprite and use that for the iconbar, distinct from 'tile_1' used in other colour 1 windows, and 'tile_m' in menus