This results in a slightly larger apk (e.g. 500KiB), but it reduces the
scope for bugs greatly. We still get all the benefits of only having to
maintain a single density-independent vector (rather than several
density dependent PNGs and all the work that involves).
The class of bugs that it solves is that there are several places where
vectors cannot be used, and you wont notice when developing on a device
newer than 5.0. For example, notification icons, `TextView`
and its `android:drawableStart` attribute.
Fixes#913.
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.
