Create "maps" directory when Widelands starts

Probably it might be a good idea to save the "official" Maps in this folder as well. Maybe in a subfolder "official". So one doesn't need to reinstall all maps if a new / development version of Widelands is installed. We might need a dialog in the installer then "It seems there are already the official maps installed. Do you want to reinstall?"

#1 This means that when staring a new game, the player will always have to navigate to the "official" directory before starting to play a new game. Also, how will the installer recognize that an official map has been updated? Do we want to replace them without prompt/install a copy/... handle this on Linux/Windows?

If your motivation is the size, maps are not the big offenders:

tribes: (248.1 MB on disk)
music: (101.1 MB on disk)
maps: (20.6 MB on disk)

We may want different folders below the maps directory?

'maps/My_Maps' for self created maps
'maps/Downloads' (or similar) for maps downloaded from the website

This may help also to find the correct location for downloaded maps, since 'Downloads' is also used in browsers.

"Downloads" would only make sense if we implemented in-game map downloads. As it is now, players can create their own an way they like. It would be a great feature to have, but we'd need more people in order to implement it.

If we have only the 'maps/' folder, i guess people will just save it there resulting in a mixed view of shipped and downloaded maps. That's not good.

If there is a folder called 'maps/Downloads/' people can use it or not. Using it would always be optional, but points a new player to the right direction, imho.

I have to admit that it is confusing showing the folder 'Downloads' in the list of maps and clicking on it will show nothing. But better than having shipped maps mixed up with downloaded maps.

Migrated to https://github.com/widelands/widelands/issues/581