Little things and breaking changes

[vypxl]

Before the 0.4 release, I want to fix some annoyances in the library that bug me.

• Systems should be just functions
• Entities should be able to have children that inherit their transform
• Fix the remaining TODOs / make issues out of them

src/scar/assets.cr: # TODO: make extendable
src/scar/assets.cr: # TODO: add more string constructors like in Components::Tilemap
src/scar/tiled_map.cr: # TODO somehow load the templates
src/scar/tiled_map.cr: # TODO load image maybe?
src/scar/tiled_map.cr: # TODO: use type field as in https://crystal-lang.org/api/1.1.1/JSON/Serializable.html#use_json_discriminator(field,mapping)-macro
src/scar/tiled_map.cr: # TODO: make data writeable (encode and compress data upon serialization) (tilelayer & chunk)

Assets should provide a default font
App should have a default scene, so no need for a useless `self << Scar::Scene.new`
App should not require implementing `update` and `render`
App should handle close events by default, but overridable