alternate-tab: Tweak pref widget a bit

Adjust spacing to use more standard values and tweak alignments
to make the structure a bit more clear.

https://bugzilla.gnome.org/show_bug.cgi?id=730843
workspace-indicator: Tweak prefs UI

 - make tree view scrollable when list grows large
 - add some borders
 - use symbolic icons instead of (deprecated) stock items
 - adjust spacing/alignment
 - disable remove button when no item is selected

https://bugzilla.gnome.org/show_bug.cgi?id=730843
auto-move: Tweak prefs UI

- make tree view scrollable when list grows large
- add some borders
- use symbolic icons instead of (deprecated) stock items
- adjust spacing/alignment
- disable remove button when no item is selected

https://bugzilla.gnome.org/show_bug.cgi?id=730843
Bump version to 3.13.2

To go along with GNOME Shell 3.13.2.