Sort the repository into folders and describe the settings once

[Solganis]

• Layout, moves only. Sources go to src/game/<module>, tests to tests/unit, tests/contracts and tests/support, loose root files to docs, config and cmake.
• Tests are registered through two CMake helpers. CMakeLists.txt drops from 661 lines to 314 with the same tests.
• One table of 27 settings in src/game/settings for the launcher and the in-game menu. A mouse sensitivity is now saved as whole thousandths.

Stacked on #36 and #37.

[Solganis]

Three more commits, the in-game menu now reads the same table.

• The menu shows a setting's description under the pointer.
• The launcher greys Smooth mouse turning without exact geometry.
• The menu greys the occlusion sliders while occlusion is off.

[Solganis]

Closing to resubmit as one pull request with the rest of the restructuring.