Don't use GtkTreeView:rules-hint (deprecated)

Replaced by the allow-rules style property, which is TRUE by default.
Add candidate-encodings gsetting

https://bugzilla.gnome.org/show_bug.cgi?id=730622
EncodingsDialog: make displayed encodings not sortable

For the displayed encodings (on the right, the encodings shown-in-menu),
there'll be up and down buttons.