Refactor editors

[snipercup]

This requires #19 to work. I will publish this PR as a draft to make sure #19 gets merged first.

This refactors the editors in the content editor to use the Gamedata autoload singleton. The primary goal is to prevent spaghetti code.

• Sprites in general are now collected from Gamedata instead of loading them from disk
• The editors only have access to 1 item now instead of the entire data array
• Data is saved by Gamedata instead of the editors themselves
• The mapeditor now uses the json_helper properly
• JSON for mobs, tiles and maps are now saved in a more readable format. The primary goal is to have the GIT version managment system recognize what lines are edited.

The code to connect the signal in the editors to Gamedata is done by the contenteditor scene. I did consider that this might be complex for other Godot users so I added some comments to explain. Since the contenteditor has access to the data array from gamedata, it passes a refrence to that array into the signal at the moment of connection. When an editor fires the signal, Gamedata will know what array to save to disk. Godot is amazing.