Add missing QTreeWidget::dropEvent to d'n'd atempt occuring in same location.

Remove 'Overwrite attempt' dialog as obsolete. Now moving item, e.g. from folder to same folder, will just make it the last child of the folder.

Conflicts:
	src/rss/rss_imp.cpp
Drag'n'Drop fixups.

1. Proper event handling
2. Allow folder moving
3. Allow moving into folders
Update changelog.