Add barebones HUD

[jonesmz]

See Magnum Integration for ImGUI here: https://doc.magnum.graphics/magnum/namespaceMagnum_1_1ImGuiIntegration.html

Requires:

1. Add Imgui as gitsubmodule, and include in cmakelists
2. Add imgui integration library from magnum as compile dependency
3. Well, actually do something with the gui library.

[jonesmz]

The work on 1. and 2. are finished, with a small step toward 3. here https://github.com/z-adams/osp-magnum/tree/imgui-integration

As such, assigning to @z-adams until he says otherwise.

[Capital-Asterisk]

Lots of changes kind of broke the momentum on this issue. Also noting that RmlUI was discussed in the discord as a long term solution.