Add support for OpenWeatherMap

OpenWeatherMaps provides free (CC-BY-SA 2.0) maps for various
kind of weather data, such as clouds, wind, temperature and
precipitation.

This is not fully functional because OWM maps are just layers,
intended to be placed above OpenStreetMaps.

https://bugzilla.gnome.org/show_bug.cgi?id=704645
ChamplainView: add support for overlay sources

OWM are layers to be rendered on top of an existing map, and
the simplest way to achieve that is to have ChamplainView render
maps from a list of sources.

https://bugzilla.gnome.org/show_bug.cgi?id=704645
Add demo of the new overlay sources

Allow to add overlay sources in the demo app.

https://bugzilla.gnome.org/show_bug.cgi?id=704645
Add a check for a correct map source in tile loading idle function

Map source can be changed before the idle is executed so we should check
whether to load the given tile or not.
Documentation and formatting changes for the overlay sources
Make overlay sources accessible outside of ChamplainView and update license actor

Add a function to access the list of overlay sources, emit the "map-source"
signal when overlay sources changed and update the license actor with
all the overlay source licenses.
view: update documentation of goto-animation props

https://bugzilla.gnome.org/show_bug.cgi?id=706282
Typo