Migrate to GTK+ 3.12.2: Bug #732129

This moves California back onto GTK+ 3.12.  Gtk.Popover use is more
spare than initially planned.  In particular, there is no popover
used that is attached to a header bar button due to a Unity bug.

Aesthetic issues remain, but to keep this patch manageable, they will
be attacked in future commits.
Increase interior padding, min width/height for DeckPopovers
Remove Cancel / Close buttons from popovers

Since user can easily dismiss popovers merely by clicking off of them,
no need for extraneous dismissal buttons.
Use symbolic icons instead of text labels in popover buttons
Restore setting initial focus when Card is mapped to Deck

This was lost in GTK+ 3.12 work.  This ensures (for example) that
the Gtk.Entry in the Quick Add popover has initial focus so user can
immediately begin typing when it appears.
Next/prev month animation should go up/down: Bug #732046

Month view animation is now vertical, week view animation remains
horizontal.  Crossfading is used to move between the views.