Add ability to Download Maps for a defined region over several Zoom levels.

Tidied up some personal code I wrote ages ago, that enables downloading of a lot of tiles at once.
Primarily intended for getting the tiles into the cache for later off line usage.

There are some hardcoded limits: 500 tiles before asking for confirmation and an absolute 5000 max.

Promote this code for public use since Marble has had this feature for while, seemingly with no complaints.

See:
http://docs.kde.org/development/en/kdeedu/marble/download-region.html
[QA] Revise direct access for '->vbox' to use gtk_dialog_get_content_area() instead.