According to the following this is not supported:
* http://stackoverflow.com/a/13471695/2391921
This uses the approach in that SO answer, by extracting the attribute to
instead point at a drawable, and have one drawable for each theme.
Fixes#979.
Still needs some better assets for the actual donate buttons, but now it includes
the relevant text about donating to developers. It also puts the donation options
in a grid layout and lets them flow across so that if there is more than three, they
will end up on the second line.
