storage-view: remove check buttons from available and used rows
These rows do not have a backing StorageViewItem - basically they are only created for UI purposes, differently from the rest that is created by using real files and directories. It does not make sense to select these rows as they can't be deleted.
Directly change the visible
property of the check buttons present in
these rows so that they become invisible. This straightforward approach
is taken because none of these rows contain a valid StorageViewItem
whose custom type could be set.
Closes: #123 (closed)