Some actions should not be started before all the previous actions
are finished. That is especially true for the actions that work
with layers stack, like "Merge Down" and "Flatten".
Now KisDelayedSaveDialog is used not only for saving, but also for
waiting before doing usual actions. It also has a busy-loop timeout of
1 second before showing up, for not distracting painters from their
workflow when the action is postponed only a little bit.
BUG:372724
Fixes T4593