modding improvements

[Grokmoo]

• Avoid sequences of anything except simple types (String, bool, number) in all data formats wherever possible. We can't currently override sequences on an item by item basis
• Ability to remove a specific value from a sequence of simple types. i.e. specify a string that should be removed from a sequence if the string exists.
• Ability to mark files as "override_only" so that they will never create a new resource, only append. this is needed for mods that work on the resources of multiple campaigns
• Mod dependencies on a specific campaign or campaigns.
• Mod dependencies on another mod or mods (submods)
• Detection of mod conflicts.
• Auto download and extract a mod to your user dir from a URL.
• Update theme to use yaml merge code and be moddable