Allow variable amount of actionbar slots

Closes #1303

Changes in menus/actionbar.txt:
- numberArea, mouseArea, and menuArea are obsolete
- Numbered slots have a new syntax. Instead of `slot1=x,y,w,h`, we use
`slot=1,x,y`. Width and height are whatever the icon size is (32 for
fantasycore)
- Like numbered slots, mouse slots don't require width and height to be
specified.