Add drawn annotations for notes/ mark-ups

[Intuos5]

It would be nice to be able to draw simple annotations to the canvas, e.g. to draw relationship lines between images, arrows or to encircle features you like from a reference image. It could be a simple vector based object like Blender's Grease Pencil.

[Cinderflame-Linear]

I took a stab at it. My PR is up... just waiting on a review.

#150