Make mod installer

[sanderd17]

It should be possible to install mods by opening a mod file with 0ad:

• The modder makes a file available for download, named something like modName.pyromod (which is just the zip generated by the archivebuilder, which includes the mod.json file)
• The player downloads the file to his downloads
• Then the player is able to open the file with 0ad/pyrogenesis
• The game places the mod zip in a new (TODO maybe allow overwriting the same mod if it is a newer version) mod folder.

Note that during the last step we can add a platform specific workaround for mods on Windows, as once a mod is mounted reading the mod.json from the zip is not possible.