Support Gnome Shell 3.10

Move to the new layout of PopupBaseMenuItem no longer column-based:
- Fix PopupBaseMenuItem-based items
- Get rid of allocation overrides (getColumnWidths() functions)

Get rid of the no longer supported comboboxes:
- Move device list to a PopupSubMenu + update Utilities.defaultUps accordingly
- Remove option to display commands in a combobox + update schema and preferences

Fix a couple of typos
Fix vertical alignment of panel label
Fix focus - now fully keyboard accessible. Fix typos.
Use new style for buttons
Add accessible names to icon-based buttons
Add Gnome Shell 3.10 support to metadata