Introduce subdirectory for object files of field buildings

These are usually very similar and not very sophisticated.
Only moved in nature.Field (so tree.yaml is not a field).
Update changelog
Translated using Weblate (French)

Currently translated at 100.0% (191 of 191 strings)
Translated using Weblate (French)

Currently translated at 100.0% (809 of 809 strings)
Translated using Weblate (French)

Currently translated at 100.0% (22 of 22 strings)
Translated using Weblate (French)

Currently translated at 100.0% (27 of 27 strings)
Rename "Iron Mine" to "Mine"

This building (like farms) is supposed to mine for many kinds of
resources, so a more generic name is a step into this direction.
Reorder a few resources, add initial brine deposit resource
Fixes #1387 - Show status icon in case settler is not connected to marketplace
Merge branch 'master' into mountains
Correct mountain graphics offset and make mountains non-constructible.