Ports Dropdown of the Overview Searchbar to GtkPopover

GtkPopover allows hiding interface elements out of the way.
The problem of click handling for dropdown menu is solved by using
GtkPopover.

https://bugzilla.gnome.org/show_bug.cgi?id=725622
search: use a consistent name for the search source enum

We prefix the other enum types with "Search", so do it here too for
consistency.

https://bugzilla.gnome.org/show_bug.cgi?id=725622
search: add a define for the stock search types

https://bugzilla.gnome.org/show_bug.cgi?id=725622
manager: add an optional action ID

We're going to use this to build GActions/GMenuModles for the various
search options.

https://bugzilla.gnome.org/show_bug.cgi?id=725622
application: add GActions for search options

https://bugzilla.gnome.org/show_bug.cgi?id=725622
searchbar: port dropdown to GMenuModel

Now that we have GActions for search options, tie it all together and
use them to build the popover contents. We have to steal the child of a
GtkPopover to get the UI, since GTK doesn't expose a public API for that
yet.

https://bugzilla.gnome.org/show_bug.cgi?id=725622