SPRITED: Support mouse access to UI elements

[dansanderson]

Version: SpritEd v0.10, as distributed with development,20231214.21,fcf8cc4

Feature request: Right now, the mouse pointer is only used for drawing. The UI implies that I can click on widget buttons, toggle, HEXP/VEXP/XWIDE and component selection elements. It'd be nice if I could click on those things, as an alternative to the keyboard.

Clicking outside of the draw area sometimes causes a click event on the draw cursor, which is counterintuitive. This would be fixed with this feature.