Change combat system from ATB to turn-based

[food-please]

This is a WIP PR.

The goal of this PR is twofold:

• Transition the combat system to be a sequential, turn-based system such as Final Fantasy 1, Dragon Quest 1, etc. The player issues orders to the characters, who then trade blows with the enemy battlers.
• Simplify the code in a number of ways:
  • The combat UI is no longer embedded in the CombatArena template scene, making arena design (should be mostly VFX, SFX, and the enemy party anyways) much easier and making the combat UI easier to find in the overall tree.
  • The combat UI is now animated by a single animation player, rather than each individual element responding to a myriad of signals. Additionally, the fade in and out and combat beginning and end are now handled in the same place as the rest of the combat start/end logic (the combat node).
  • Combat menus no longer fade in and out. Response times should be snappier and there's no need to inflate and convolute the code by await-ing for menu animations.