add eventloop in ProgressManager to wait progressitem deletion is complete...

add eventloop in ProgressManager to wait progressitem deletion is complete when slotAbortAll() is called.
By this way, if user close digiKam when progressitems are currently running in background (as some maintenance tools),
this prevent violent crash due to close remain active objects in memory.
BUGS: 302810
FIXED-IN: 4.0.0