Blender Gui Widget update

[lisovykzlisy]

Hi. I`m using blender with my custom plugins. Though the work I found I need more complicated GUI features not included in default branch Blender Foundation. So I find the great work of Jayanam, but it far from ideal.

Changes: Fix of crash GUI with changing layout. Added check if is active the area assigned in initialization. Fix of crashing with undo after calling operator from custom gui button function. Avery execute event creates Blender undo control point. Added new widget "Background" just for fun and testing.