Support multiple translation files for mod support

[bitcraft]

NOTE: this is a discussion on a feature in development and is likely going to change as it is developed. This is related to mod support. Mods are stored in the the "mods" folder, with the default being "tuxemon". This feature should allow translations of the same locale to be loaded from multiple folders. When this is implemented, a number of issues must be handled:

• The order of mods must be explicit. For example, a mod might want to override something in the default campaign/mod
• There should be logging output for a developer to verify that the overrides are correct
• The PO translation code (generating MO files) must support multiple folders
• There must be a "mod loader" which scans and verifies mods

[MirceaKitsune]

Just a clarification as I forgot to post this a few days ago: #601 should half fix this. The new game loader allows all files to be overridden from a campaign's directory, including locale. So if you have a mod/mygame/l18n/en_US/LC_MESSAGES/base.po it will replace the default resources/l18n/en_US/LC_MESSAGES/base.po if you're launching Tuxemon with the mygame data folder.

Like I mentioned the only slight critique I have is that one base.po contains everything from dialogue to builtin command names, instead of being divided into a small category of files (eg: builtin.po, defs.po, dialogue.po) which would allow a campaign to only modify and maintain the ones it needs to. Other than that I'm personally satisfied with how this will work following that PR.

[bitcraft]

There are features which still need to be implemented so I will keep this issue open. Please keep the “split PO files” comments to its own discussion.