Add PlaceEntry, a SearchEntry for places

PlaceEntry encapsulates all the search entry stuff in mainWindow in a
reusable way.

https://bugzilla.gnome.org/show_bug.cgi?id=731545
MainWindow: use PlaceEntry

Move over to using PlaceEntry in the HeaderBar.

https://bugzilla.gnome.org/show_bug.cgi?id=731545