This uses gdiplus to convert the PNG data. It also
simplyfies the invoke as the size is now taken
from the resource file.

* src/Makefile.am: Link gdiplus
* src/gpgoladdin.cpp (Invoke): Just call getIcon for button
callbacks
* src/ribbon-callbacks.cpp (getIcon): Load images from resource
as PNG and convert them.
* src/ribbon-callbacks.h: Use png icons.