Link projects to share common assets and scripts

The copy/paste feature is a god sent! Thank you all who made it happen :)

With the new feature in place, a new behavior patterns emerged. One such is to develop a common framework or an SDK, that holds scripts with convenience methods and a template scene layout with screens and all. When a new project starts, we copy/paste a framework to the new project, so there is a boilerplate, and as we develop the project, we add to the original boilerplate additional common methods.

When a new project starts, we grab the framework from the latest project, since it has new methods and stuff, remove the pieces not needed and use it in the new project. The cycle goes on.

This creates diffferent versions of potentially the same framework. It would be super nice, if there was an option to link a project. Like you link libraries and projects in Microsoft Visual Studio. This would allow to always point to the latest version of the framework which would be maintained in its own project.

The issue names no files, tests, or entry points. Start by locating the editor's project and asset-management entry points and any existing library-linking behavior. Done would mean a project can reference a separate framework project so its shared scripts and scene assets stay available from the latest version.