placeEntry: Add 'loupe' property

Add a property that controls whether to show the loupe
icon in the GtkSearchEntry or not.

https://bugzilla.gnome.org/show_bug.cgi?id=733554
Only use the loupe icon on the main search

Do not display the loupe icon on the route planning
PlaceEntries.

https://bugzilla.gnome.org/show_bug.cgi?id=733554