Now we only call Pathfinder.find_path() when clicking on empty cells

To know when an encounter starts and connect to newly created monsters in CombatArena

This is a breaking change. I'm looking for some feedback on how the code feels now.

Quests start but won't complete yet at this point. I
want to do #167 and hopefully #168 as well to simplify the system even more.
Until now it was a pain to just create quests and place them in the game world,
for multiple reasons. E.g. The fact that the GiveQuestAction is inherited in
PawnQuestGiver scenes makes it so the quest reference gets lost every time you
modify the action's script in all instances.

The Journal is also broken right now!

- Made the QuestSystem as a singleton: it had to be passed around multiple parts
  of the game, and probably more and more files as we expand it. I'd rather
  avoid singletons but as the API is limited this has the potential to simplify
  the code IMO
- Devs should now ONLY interact with QuestSystem. The API is a lot simpler:
  start() and deliver() to start and finish quests talking to NPCs. For now the
  rest is handled by the quests internally, based on their objectives.

Removed QuestPawn
Made Quest bubble standalone
Added docstrings to a few nodes

I left some TODO notes for more things to refactor. I think the questsystem api
still has some room for simplification.
Every NPC should also be able to handle quests imo

Quest journal added

Pawns on `local_map` now have a "bubble" on top of them that indicates that they have a quest to give/active/to be delivered by players. 

Works with both:
- Deliver quest to the same NPC that gave it
- Deliver to a different NPC

Gilles explained the UndoRedo object is shared by the entire editor, so another
plugin or function could write to the object in between our do/undo actions,
causing bugs or crashes.

Quest rewards added

I've really got to take some rest 😂

fix wrong node path in DebugInterface

Trying to use Resource to gather all the save data and let Godot store it on the
disk for us
This makes it possible to edit savegames from the inspector
Added a DebugInterface to save and load data, for testing purpose

Basic quest system added

Refactor GrowthStats out of CharacterStats

set initial animation for PawnAnim