[upstream utils] Upstream imgui and friends

[pjreiniger]

Fixes #6809

• Gradle build
• Cmake Build
• Publishing

I made some executive decisions for this first pass.

• Libraries are split into include / src, like is standard here, but was not in the thirdparty repo
• I'm building the imgui / glfw / gl3w / stb / fonts as a single library, like the thirdparty repo. Because of this, the upstream file bundles them all into a "imgui suite"
• The glfw sources are left as .c, instead of getting automatically converted to .cpp like the upstream library wants to do by default. These are in fact C files, and are compiled as such, so I think their extension should match.
• I'm treating fonts and gl3w as generated files as opposed to part of the upstream process. This more accurately reflects how things are built in reality, and what happened in the thirdparty repo. I copied the generate_fonts.sh script as, so it is a bash script that will only run on Linux

[calcmogul]

This needs to be rebased to use the new upstream_utils API.