Prevents a control's clearBounds from having a width or height less than zero.
Disables selective container clearing as it can cause parts of a form not to be drawn.  We can turn this back on later with a little bit of work.
Fixes #877

Controls only need to be ordered by Z-Order when within an absolute layout.
For all other layouts created so far, controls should never overlap, and their order should be kept as it was created.
Merge pull request #885 from ablake/next

Small UI bug fixes.