One more try to fix restriction for adding masks to the root layer

We should not allow adding a mask to normal root layers. It makes
painters confused. But for clipboard contents is it perfectly okay,
so just add an option for it.

It the previous version the check for the presence of a global
selection mask was incorrect.

BUG:397796
BUG:407375
BUG:407665

BACKPORT:krita/4.2