Add distinct systems and clean up

* Fix misleading y-offset in font mesh builder
* Add collision system
* Add collision groups
* Add basic parent/child relations between entities
* Add particle system
* Add basic particle emitter
* Add coefficient of restitution to impulse based collision model
* Remove ability to transform meshes while loading (avoid duplicates)
* GUI elements are now calculated in fixed size and in [0, 1] space (units are still misleading)