[4.0] Create new .sln file for all new projects

[FreezyLemon]

Purpose of this PR

• Move all new projects that were/will be added during the development of version 4.0 to a new OpenTK.sln
• Leave the old .sln available as OpenTK.Legacy.sln (just to access code, build is broken now)
  • Should be removed when 4.0 is done/everything is ported or updated
• Move .props files to props folder (old structure was named incorrectly (those aren't build targets) and unnecessarily nested)

Comments

This should allow us to work on 4.0 more easily, e.g. to:

• Fix stylecop errors (no need to do so in the old projects)
• not break the new projects
• get a new build process up and running (preferrably without FAKE)
• get CI working again

and so on.

[Nihlus]

Looks good.