This widget is far from finished, but it's off to a good start.
Also added a GResizeEvent and GWidget::resize_event() so that widgets
can react to being resized.
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.
