Introduce an AnglePropertyEditor

Summary

It's a somewhat common case to specify angles in the inspector. However, radian angles as used throughout Duality are not easily readable by humans. There should be a specialized property editor for angles that allows to switch between radians and degrees and defaults to degrees.

Analysis

• Note that issue #528 is a prerequisite to this.
• The editor should feel like an "extended numeric editor", so nothing entirely new. Same float field, limits, dragdrop, etc.
• There should be a unit label (deg / rad) with a button area of some sort for switching units.