Blender Git Loki

GP Eraser Panel: Expose button to activate eraser

It's currently impossible to activate the eraser in the 3D view otherwise
if you're using a mouse (unlikely for most serious 2D artists, but possible
for non-sculpting modellers/animators) or if your tablet doesn't have an
eraser end (IIRC some non-Wacom styluses may have this limitation)

Also noted that we should also expose the hard/soft mode, and/or strength
settings here too (as opposed to having these hardcoded and/or mapped to
hardcoded modifier keys only)