XBM icon support

<icon=/path/to/icon>
Some tabs slipped in, reindent
Add finalizer for the icon pixmap
reindent again
Implement icon caching
Move bitmap functions into Bitmap module
Fixes for previous
<icon=..> -> <icon=../>
Add standard header
Fit to 80 columns
Slight hlinting
A bit of documentation and credits for Edward and Alexander
Remove unsafe fromJust from Volume plugin