Add dependencies for mods

[cech12]

Add a possibility to let mods depend on each other. A mod should be able to define other mods, that must be installed before (required or optional).

In the moment of adding a mod to the active mods:

• all required mods should also be added
• the mod should be arranged below all required and optional mods
• if it is an optional/required mod of another active mod, it should be arranged before it.
• (circular dependencies?)

Could be combined with the hint/warning system: #34