FR: Add gestalt leveling tracks

[ChronosCrake]

Add an option to the class screen to add a second leveling track to allow for gestalt progression. Take the highest value for BAB, saves, hp, and skill points at each level, and make class skills apply from both levels simultaneously, as well as granting class features for both gestalt tracks.

Mock up of what the level progression screen could look like:

Clicking level 1 on either track would open up a slightly revised level select window, perhaps something like this?

[github-actions[bot]]

Thanks for opening an issue, traveller! If your issue is related to an actor or an item, sending thing that is broken will help a lot and speed up debugging.

How to send an item? > - If item is owned by a character, drag and drop it to world item list. > - Right click on item in browser item list and click export. > - Attach downloaded item to this ticket. How to send an actor? > - Right click on actor in browser actor list and click export. > - Attach downloaded actor to this ticket.