region: Make it possible to add an input source by activating its row

Selecting and pressing the Add button continues to work but this
should streamline the the input chooser dialog for mouse users by
making it possible to add by double clicking a row.

https://bugzilla.gnome.org/show_bug.cgi?id=732720
region: Add Cancel/OK buttons to the format chooser

These were lost with the transition to headerbars.

https://bugzilla.gnome.org/show_bug.cgi?id=732725
common: Add Cancel/OK buttons to the language chooser

These were lost with the transition to headerbars.

https://bugzilla.gnome.org/show_bug.cgi?id=732725