[WIP] Add new interface elements to represent the combat turn order (#118)

* [WIP] Add new interface elements to represent the combat turn order...

This commit add two new scenes: TurnOrder and CombatPortrait.

The TurnOrder scene is designed to be a child of the CombatInterface node in
the CombatArena scene. It is initialized at the same time as the other
interface nodes, and dynamically display a CombatPortrait node for each
battlers, both PC and NPC.

For the moment, the CombatPortrait are just placeholders, and do not display
the real battlers portraits.

* [WIP] Rebuild the TurnOrder interface every time the TurnQueue is updated

* [WIP] Better CombatPortrait animation between combat turns

* [WIP] The TurnOrder node manages its own connections with signals...

... also add missing docstrings, and a CombatPortrait placeholder

* Fix previous commit, add missing Godette combat portrait

* Do not rebuild the TurnOrder interface if no battler enters of leaves the arena

... also add better combat portrait for Godette, Robi and Porcupine (256x256px)

* Animate the CombatPortrait faster

* Finally add the combat portrait of each battler

* Use battler.is_able_to_play() in CombatArena instead of directly checking the stats attribute...

... and minor comment changes in the turn order interface.

* Slightly reduce the space between CombatPortrait in the TurnOrder interface