Fix zoom type definition in map cache.

Viking zoom levels can be negative so change from guint to gint.
Prevent crashing when queue_tail is NULL.

This can be induced if the map cache size is very low (e.g. 1Mb),
 such that the queue has no values.
[QA] Rename and correct some map cache variables usage for better understanding.
Add map cache information to be accessible from the GUI only in debug mode.
Add link to HERE Maps.
Add capability to define the URL format for web links.

Enable link to Geofabrik Map Compare.
Enable i18n on 'Routing' preferences tab name.
[QA] Window's build - better use of environment variable.