Emit aboutToEmptyCache to invoke RadialMap::Widget::invalidate(),
so folder can be safely deleted.
If "Scan across system boundaries" isn't set and there is mountpoint B
contained in another mountpoint A, user may trigger segfault if scan of
A and then B is requested. It is caused by call of MainWindow class to
m_map->invalidate() method, which is invoked after m_map's m_tree member
is deleted during ScanManager::start method.
This fix causes that RadialMap::Widget::invalidate() is called twice in
fact, however second call is fine, because "invalidate" is guarded by
"isValid" method.
BUG: 389119