Unify Lua modding under GameMode (Redux)

[ajhalme]

Attempt to merge @esbudylin's Lua consolidation effort on top of latest Development branch – a continuation of #906.

My plan was to take all the new abstractions, but leave the Lua code where it is for now, but I'm not sure that's viable. If there's appetite, we could just go for the whole Lua migration and cleanup afterwards.

All the PediaIcons code looks really shaky. I couldn't immediately see how to fix Standalone mode Load Game, but New Game seems to work at least.

[stavrosfa]

To be totally honest, I don't feel really comfortable with this.

I think we should first ask @esbudylin if they intend to finish this, (or it's done and just needs some documentation or something), and then we can see our options.

My biggest fear is that this is a huge PR, I haven't really looked into it, because I was expecting the extra documentation Yegor mentioned, and even if I do, I am sure we can expect some disruptions that are natural with this type of changes, which will take a lot of time to debug, especially without the support of the original author.

If the codebase was in a better and more stable state than it currently is, I would be more inclined to plow through this.

But, even with my newer commits, as well as with the "older" codebase, there are major bugs (old and introduced) that are in constant need to be solved and refined, and I feel like we should be putting our efforts there (and this is what I am trying to do), or at least, not modify a huge chunk of the current codebase.

@WildWeazel I would like your input on this too please

[WildWeazel]

Popping in without context yet- has Yegor talked about where he was going with this, or was there discussion about how to wrap it up? And is this currently blocking other progress? The mod interface is one of those things I want to make sure we "get right" since it will be such a pivotal part for other people to build on, so in general I'd favor taking time to get a consensus, even from non-dev modders at times. I'll try to catch up on this over the weekend.