Allow copying/moving files from one project to another in the project tree view.

The commit adds another method to the IProjectFileManager to copy external files into the project, which custom project managers can override. It also adds a default implementation to the AbstractFileManager class, which just does a plain copy (i.e. using the copyFilesAndFolders method).

Moving is done by copying first and removing the source afterwards. A simple move is not, in general, possible, since e.g. moving a file from one git repository to another is not a supported operation.

BUG: 386947