Blender Git Loki

GPencil Keymap: Pen Eraser can now be used for erasing strokes

Using the keymap support added in the previous commit, we can now map the
pen's eraser to trigger erasing.

Pen drawing support is not enabled though, since users who use a tablet as
their primary pointing device will want to be able to continue using it
for things like selection, grabbing manipulators, and sculpting/texture painting.