Fix a crash when opening a folder, and another one when trying to open
a newly created folder.
It was not safe to modify a GModelSelection while it's being iterated over.
Fixes#536.
Each GAbstractView now has a GModelSelection backed by a simple
HashTable<GModelIndex>. When the selection changes somehow, the view
gets notified via the notify_selection_changed() callback.
In the future it will probably make sense to move to using some kind of
ranges as the internal representation instead.
