Blender Git Loki

Bring mask tools back to Masking Tool Area and update Context Menu

The mask tools in the Tool Area of Clip Editor where never brought back,
after decision was made to postpone proper masking tools to 2.81.

This brings back the Mask Tool Panel in the Mask tab of Masking Tool Area.

This Diff also adds a better Context Menu, which respects whether you're
in Masking or Tracking mode. I have added the most important operators
there, with a focus on those that are otherwise harder to access.

Reviewers: sergey, billreynish

Subscribers: sebastian, brecht, hype

Tags: #motion_tracking

Differential Revision: https://developer.blender.org/D5075