Implement ECS-friendly GUI

[Mirsario]

The current (barely existing) GUI implementation is just leftovers from the old Unity-like Entity-Component design, in which every (programmable) component could override an OnGUI method, to then use static immediate GUI rendering methods in there. This is, of course, quite horrible. All the code for that should be removed, and replaced with an ECS-friendly design, in which all GUI will work with the help of specialized components.

[Mirsario]

A nice and helpful gist on the topic of ECS GUI that could be used as a reference:
https://gist.github.com/MrSmith33/f210a7cbd79f6c5446a93dd044ec371e