Implement MappingTreeWidget

This is a rather large commit. It implements MappingTreeWidget using
MappingTreeModel unsurprisingly this uncovered some exciting bugs.

Fixes the following bugs in MappingTreeModel:
* use insertRows() and removeRows() when addings children, because
dataChanged() won't do it.
* don't use a prepared QSqlQuery when fetching children recursively.
This won't work because the query is still active when we invoke
ourselves again. Put the query on the stack instead
* Keep the model sorted.

Also add an entry for a MappingTreeEditor to the File-Menu.

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