Disconnect "cursor_changed" when clearing the treeview and make on_change() stop if the selection is empty. Fixes #11.
Merge pull request #28 from Falc/issue-11

Fix #11
Don't use default values for abstract properties in interfaces. Fixes #26.
Merge pull request #29 from Falc/issue-26

Fix #26
Imagesets should be cleared only if they have been instanced. Fixes #27.
Merge pull request #30 from Falc/issue-27

Fix #27
Sync with upstream