Support showing location messages

[marhkb]

[marhkb]

Now with avatar map markers for live locations.

[melix99]

After trying this is my touchscreen laptop I've realized that it's not a good idea to make the map navigable, because it interferes with the chat history scrolling while using touch. I think that the MessageMap widget should just be a sort of preview of the position, which can then be clicked to open a full map that can be navigated. Since the last bit it's a bit more involved, I suggest to just focus on a MessageMap for this PR.

[marhkb]

The map is now insensitive. Besides, the OSM copyright text has been adapted to the new AdwAboutDialog.

[alissonlauffer]

Just wondering if it could be added support for Venues too (Venues are almost the same as locations, but they come with a place name too instead of just an arbitrary location):

[marhkb]

Just wondering if it could be added support for Venues too (Venues are almost the same as locations, but they come with a place name too instead of just an arbitrary location):

Yes, I think this would be quite easy. But I would rather do that in a separate PR before this becomes too big.

[melix99]

I've marked this as draft, it needs to be ported to the new MessageBubble widget. I've set the milestone for 0.2 for now, but I might change it back to 0.1 if it gets ported in time.