Master issue for general container functionality

[andlabs]

Combining a bunch of disparate issues here.

• Deleting a control from its parent through the control itself rather than the parent (maybe) (see Removing controls from parents #41 for rationale)
• Consistent behavior of NULL children, including on single-child containers
• uiGrid behavior when asking to add a cell that is already occupied (already exists) or remove one that isn't (doesn't)

[andlabs]

Hm, I thought I had more to put here...

[mimecorg]

I'm implementing Vuido which is based on libui and one thing that I miss is the lack of an insert method for containers. I had to work around this by removing and re-appending all children following the one I need to insert.

[mischnic]

+1 for uiBoxInsertAt and uiBoxRemoveAt