A more radical approach to inset background painting

A more radical approach to inset background painting

Now by default all insets paint their own background when needed. This
means that 63cf3297 and part of 9940acc5 can be reverted.

To avoid extra painting, background drawing is disabled for
InsetCommand and InsetCollapsable. These insets draw background as
part of their normal drawing activity.

This will avoid drawing artifacts with InsetNewpage, InsetVSpace and
probably some others.