Add add_location action

Add an action to add a location to world. The parameter is
a serialized GWeather.Location which is unfortunate but for the
moment might be the best.

This is needed to allow other applications, such as Maps, to share
locations with us.

https://bugzilla.gnome.org/show_bug.cgi?id=740356
world: Do not add duplicate locations

Add checks to see if the location already exists before adding.
In the case of adding through the 'add-location' action just
silently omit to add.

When adding through the LocationDialog do not allow adding
if the location exists.

https://bugzilla.gnome.org/show_bug.cgi?id=740356