[ATLAS] Improved mod support

[historic_bruno]

Atlas should have better mod support. This includes:

• Map loading/saving. Currently Atlas uses a standard file open/save dialog which is misleading, it doesn't really access files directly and ignores the paths seen in the dialog. Instead it should be redesigned to work around the concept of mods. See #889, #631.
• Mod selection. The user can select one or more mods to use for e.g. scenario editing, but a single mod for writing changes (see also #1940).
• Mod packaging. Mods can be packaged into a zip for sharing with others.
• Mod editor. A mod editor would show all the files in the current mod, allow adding, removing, or in some cases editing them (actors, scenarios, etc.)