Add a default testing / sandbox game plugin project to the main solution

[ilexp]

Summary

With the new, "reverse" dev environment, the Duality editor is no longer in charge of creating or maintaining plugin source code, projects or solution files. However, this also means that the dev environment of Duality itself no longer has an easy to use sandbox for quickly testing some code.

Analysis

• Up to Duality v3.x, you could just compile the solution, run the editor and click "Open Source Code" and it would create a new game plugin from scratch that you could use for testing. This is no longer the case.
• The dev solution already contains a launcher project that's there purely for testing purposes, as a replacement for the one otherwise generated by user code in v4.0.
  
• Maybe it would make sense to also add a "GameTestPlugin" project next to it, which is empty except for the core plugin definition and a test component. This would serve as a place where devs can easily write some test code.
• The "Launcher" folder could be renamed to "TestProject", which would then contain both the launcher, the "GameTestPlugin" (and, potentially an "EditorTestPlugin" later on at some point)