Integrate ImGui (imgui-rs) into Tetra as a early GUI solution (Fix issue #310)

[Nekomaru-PKU]

I've made my changes ready for being merged into the main branch. Example usages and other information can be found under https://github.com/17cupsofcoffee/tetra/issues/310

[17cupsofcoffee]

Hi - thank you for the effort you've put into this!

That being said, I don't think this is the sort of thing I'd want to merge into the main branch of Tetra, for a few reasons:

• I don't want to favour one particular UI library over another by integrating it into the core library. There's other popular options like egui out there, so this change feels like it's a bit too opinionated.
• I'm not sure how well the approach of using imgui_glow_renderer would work - Tetra's graphics backend code does a lot of GL state caching, and having an external library mess around with the OpenGL state could very easily break that.
• Tetra is no longer under active development, and I don't have the time to support a big change like this through to a release at the moment.

My recommendation would be to create an external library for this kind of integration, like egui-tetra did - I got halfway through doing something similar for imgui last year, but didn't have the time to finish it off.

Sorry for the disappointing reply, I really hate to turn down pull requests that have clearly had a lot of effort and thought put into them 😥