Another polygon improvement from Issue #1907

[WileECoder]

This PR addresses issue #1907 and improves the behaviour of Bezier polygon (and the Mask node).
Both issues from @itsmattkc and @unfa are addressed; in particular:

• a set of points can be selected, and more points can be added or removed to selection
• selection can be edited by clicking single points or with a lasso selection
• clicking inside the polygon moves the whole polygon, consistently with what happens for Text and Shape
• only handles for selected points are shown
• there is a visual feedback when a gizmo is hovered and ...
• ... points are bigger and easier to select
• if a Bezier point and its control point are in the same position, dragging always affects the main point.
  In order to grab a control point, ALT modifier can be used. If control point is isolated, ALT modifier is not needed.

I have made a video to show how this works.

NOTE: This PR duplicates the one from @p9malino26 (#2030). I did not realize he was working on the same issue.
Feel free to use the best implementation or take something good from both ...

[ThomasWilshaw]

Olive is about to undergo some largish code refactoring and I wanted to check on the status of old PRs before we do that. Do you plan to update/continue with the PR? Thanks

[WileECoder]

While I still think this is a useful feature, chances are that a refactoring will generate too many conflicts. Fell free to close this.