Cleanup and fixup ProjectUtils to not leak and not trigger crashes.

The most important part of this patch is the change to use a dynamic
allocated QMenu which is then _not_ run synchronously via exec() but
rather allocate it on the heap and use QObject/QWidget parentship
to handle the deallocation eventually.

This also fixes the memory leak of the populator objects.

And it fixes a crash since QMenu::exec applies a nested event loop
which apparently does not work nicely together with KXMLGUI. One
could also fix that by delaying (i.e. Qt::QueuedConnection) the
handling of _all_ QMenu actions - but that won't work of course.

So yeah, I think this is the best option to handle this crash.

Maybe one should even investigate whether we can/should deprecate
the usage of QMenu::exec in our codebase...

BUG: 311447