This logic is all taken care of by GAbstractColumnView now, so we can
simply delete GTreeView::context_menu_event(). :^)
Fixes an issue mentioned in #826
This makes GTreeView able to support multi-column models!
Only one column can be the "tree column", this is column 0 by default
but can be changed by overriding GModel::tree_column().
When the filesystem model is updated, it is rebuilt. This means dangling
indexes inside the TreeView metadata table will have old information and random
directories will toggle open. Clearing the table alleviates this issue.
GTreeView was forgetting to call to base in did_update_selection().
This prevented GAbstractView from firing the on_selection hook and we
ended up with only the on_selection_change hook firing.
Previously it was only possible to have a single root-level item in a
GTreeView. This was an oversight and I didn't realize it because this
code was only ever used in the FileManager, which has one root ("/").
Also factored out item toggling into a separate function, and increase
the base indentation level so that root items can be toggled as well.
Finally, let the user toggle the selected item with the spacebar. :^)
