Explicitly spell out the long-term development plans

[captainurist]

Describe the Idea This has been discussed in Discord, but shouldn't really stay in discord.

Several points need to be explicitly spelled out:

1. Dependency structure, and "what goes where".

E.g. we had this chart in Discord:

      /         Application        \       deps
     /        Engine|Arcomage       \       | 
    /           GUI|Media|IO         \      |
   /             Platform             \     |
  /              Utility               \    |
 /              thirdparty              \   |
/           external libraries           \  v

Where do generic libraries like compression algos go in this picture?

2. Class hierarchy / structure. A rough sketch should be enough for now.

3. Stretch goals.

[captainurist]

See updated milestones.

Will update HACKING and can then close this issue.