System to automatically add new projects to solution in content

[PJB3005]

With the way RT and content assemblies are configured, any addition of a new project in RT is a breaking change that needs the solution to be updated in content. This sucks!

The solution format does not support any form of "import this other file" like MSBuild does. So that really leaves us no option for doing this with "standard" tooling.

My current idea is to make RUN_THIS.py automatically splice things into the solution file. Given that git_helper.py already runs every time you run a git checkout by default, this should be able to work? But it might be annoying.