Blender Git Loki

GPencil: Add hotkeys for the "Delete Active Frame" operator

Usage:
* D+X - Works anytime, anywhere
* Shift-X - Works in EditMode only
* Via Delete Menu - EditMode only

Often doing video tutorials or perhaps during dailies/shot review you want to
quickly get rid of a quick scribble you made for making a point, without having
to undo (i.e. maybe you edited some objects in between) and/or without having
to use the eraser (i.e. it'd take too long to cover the entire area).