file chooser: Fix multi-selection

file chooser: Fix multi-selection

Even if we only ever hit the code with a singleton selection,
calling gtk_tree_selection_get_selected is not ok if the tree
selection mode allows multi-selection. Replace all calls to
gtk_tree_selection_get_selected in the file chooser code with
callback loops iterating over the selection. This problem
was introduced with the recently added rename and delete
menuitems.