Editor: Merge & Unify right-click lists on toolbar buttons
Some toolbar button present the user with a dynamic list of text items when clicking with the right mouse button (currently terrain texture brush button and run button).
The implementation of these lists should be unified into a single class (either fully configurable or as base class). This will enforce consistent look and functionality (brush list currently missing remove button) and also allows for code re-use when more of these lists are added.
(RM-4440 from redmine: created on 2018-06-04 by Marc Zinnschlag, )