Move search result model knowledge to SearchPopup

This allows easy use of the SearchPopup from more places than
mainwindow.

https://bugzilla.gnome.org/show_bug.cgi?id=723895
Move completion match function to PlaceStore

This enables the use of the PlaceStore for entry completion in more
places than MainWindow.

https://bugzilla.gnome.org/show_bug.cgi?id=723895