Because the logic is in rename_tag() now, it means we create a
checkpoint even if the tag is orphaned. This is because currently
checkpointing is a GUI responsibility. In the future we need to introduce
multi-level undo, and should move responsibility for managing it
to the backend.
- Move the new sidebar code and SidebarTreeView implementation to a
new file. Old sidebar can be accessed with the shift key. The "new" sidebar
doesn't add much for now.
- Make rename_deck work with subdecks
- Fix an issue when trying to open context menu over areas not populated
with sidebar items
