Let GButton have an optional icon (GraphicsBitmap) that gets rendered in the
middle of the button if present.
Also add GraphicsBitmap::load_from_file() which allows mmap'ed RGBA32 files.
I wrote a little program to take "raw" files from GIMP and swizzle them into
the correct byte order.
This only works with the userspace build of SharedGraphics so far.
It's also very slow at loading fonts, but that's easy to fix.
Let's put fonts in /res/fonts/.
