Allow executing multiple timelines at once

[Jowan-Spooner]

Right now all the timeline execution is done by the dialogic game handler. While for many games one timeline at a time is the most logical thing to do, sometimes executing multiple timelines might be desired.

This would likely mean the following:

• move much of the timeline execution into a new "TimelineHandler" class
• make the dgh instance this class when you do start()

There is a couple of problems though:

• Information is currently universal (e.g. current_state_info). Even if each timeline handler had it's own state info, the subsystems and events would have to know which one to use.
• If you run multiple timelines at once, you likely want them to be displayed at the same time as well. However currently the DialogicNodes and Subsystems have no way of doing that. This could be solved by giving each dialogic node a layer (like visibility or shadows) and then give each timelinehandler a layer as well, so it will only target dialogic nodes of that layer. But not sure if that is the best way to do it or overkill.

Need to think about this more. Also I don't really know how needed this is, as I can only imagine some usecases for this. What could be way more useful is to allow stopping a timeline at any point and resuming the timeline later from there (like if you go away from a conversation and go to another one but later come back).

Just a big rant that serves as a note for later ;).

[ZenithStar]

Bumping this issue following a thread on the discord.

My use case for this feature to is implement both voice and textbubble "barks" (e.g. in Unity) as Dialogic timelines. For example, in an RPG, as you walk through a town, various NPCs will perform short dialogue lines as you walk by them. These dialogue lines should be able to be performed concurrently. If an NPC was in the middle of delivering a bark and the player walked up and interacted with them, they would finish the bark line before moving onto the rest of the dialogue. If the player decides to leave an interactive dialogue and regain movement control, the NPC wouldn't just cut off, but finish the dialogue line while the player moves away.