[Feat] ImGui + ImGui Game Console

[ashifolfi]

This PR adds support for using ImGui alongside an implementation of the Source Engine GameConsole in ImGui.

This uses the VGui implementation of ImGui and has the main ImGui implementation stored within a static library called devui. The GameConsole code will be contained within game/client/devui_panels as I do not know where else to put it at this time.

(Rough List) Tasks:

• Add ImGui System target/lib
• Add ImGui System to Client DLL
• Make sure all parts of this setup compile properly
• Implement ImGui GameConsole over VGui GameConsole

Currently none of this is compiling because VS is giving me grief.

[ashifolfi]

The odd setup of baking the ImGui source files into devui is how the implementation I have in my private fork functions. You are free to figure out a better way of setting that all up. I am merely attempting to copy my setup with a few improvements at this time.

I originally just baked the devui files into the main client DLL rather than using a static library.