Skip paint event when in the middle of a buffer operation

Skip paint event when in the middle of a buffer operation

This is detected when an undo group is open and contains at east one
element. This means indeed that changes are in progress. Note that the
group is in general opened in GuiApplication::dispatch. The code there
is changed to ensure that the group is closed before updating the
screen.

This patch is experimental. It is expected to be replaced in master by
a more complete solution. It could in the meantime be backported to 2.3.x.

Fixes bug #11159.