Improvements to Dialog_NodeTree

[SokyranTheDragon]

How we currently handle Dialog_NodeTree windows could use a bit of improvements. Currently, we have PersistentDialogs file with classes related to them, but it doesn't really seem to me like we're doing much with them. The file:

https://github.com/rwmt/Multiplayer/blob/development/Source/Client/Persistent/PersistentDialogs.cs

The improvements they could use:

• [x] Make them non-blocking like trading, caravan forming, rituals, etc.
• [x] Use them for vanilla features using those types of dialogs (that don't use it already), like Caravan Meeting and Caravan Demand (and remove the current sync from them, as it's a hacky solution I implemented without realizing about the existing implementations)
• [ ] Expose their use in the MP API so mods can benefit from those features
• [ ] Use those changes to sync ChoiceLetter classes, like accepting pawn request to join - this would fix #241, #104, and #256

[SokyranTheDragon]

Just to answer potential questions about this - yes, I'm working on it right now. I've made this issue to keep track of the progress. I'll most likely create a draft PR somewhere tomorrow, which implements some of those changes