Blender Git Loki

Annotations: Add different arrow styles for line tool

This patch adds different kind of shapes/styles for the line extremes while using the annotation line tool.

Current Styles: (following @mendio mockup)
- Arrow (closed arrow)
- Open Arrow
- Segment
- Square
For future it would be great to have icons, it would be more intuitive (and less space) with previews of what each end / start of line does, like the google slides one as reference:

{F8511116}

Reviewed By: #grease_pencil, antoniov, HooglyBoogly

Differential Revision: https://developer.blender.org/D7608