[Request] Add ability to create custom Entity presets

[mundusnine]

Idealy, Entity presets could be saved in the .ldtk file directly.

Also, it would be cool to be able to change the preset fields and automatically add the new fields to entities using the preset in question.

For example, we have an Enemy preset that starts off with a health and strength field. We create an OrcSwordsman and OrcMage. We then realise we need a range field or powers field, we add it to the preset and it adds it to the already created entities(OrcSwordsman ,OrcMage).

[deepnight]

Related to #408