Support direct offline map download links

[rsudev]

When recently discussing the current evolution of c:geo mapsforge support the maintainers of openandromaps suggested that c:geo should support a special url scheme to trigger download of map files from their site. Besides the orux and locus specific ones they do have 'generic' ones we could tie into. This sounds to me like an easier to implement first step for #6072

[rsudev]

Link to the forum discussion: https://www.openandromaps.org/oam-forums/topic/cgeo-unterstuetzt-elevate-4-und-aktuelle-kartenformate#post-28215

[Lineflyer]

On http://www.freizeitkarte-osm.de/android/de/impressum.html
I have also just seen, that they support a XML repository to load files and themes.

Edit:
I think this would rather be useful for #6072 and is already mentioned there.

[moving-bits]

@rsudev @Lineflyer
Still need this issue after the recent implementation of the OSM map downloader? (see #8610)

[Lineflyer]

No longer very important but still a thing which could be evaluated as advanced option.

[moving-bits]

FYI: c:geo has full OpenAndroMaps download support meanwhile:

• add OpenAndroMaps as source for MapDownloader #9851 (map downloader support for OAM)
• add support for mf-v4-map scheme for downloading map files (fix #9918) #9921 (map scheme trigger)
• support mf-theme scheme for downloading map themes (fix #9920) #9945 (theme scheme trigger)

Regarding freizeitkarte: From reading their "Impressum" I guess we are free to use their maps in our downloader. I'll put that on my TODO list.