Blender Git Loki

UI: Add Missing UV Unwrap Operators to UV Editor

This commit also changes the U shortcut to open the unwrap menu instead
of the Unwrap operator. The unwrap operator can now be accessed by
pressing U twice.

Note, these operators use the 3D Viewports selection and not the UV
Editor selection. In the future the operators should unwrap based on the
selection within that editor.

Fixes T80600

Differential Revision: https://developer.blender.org/D8834