enable ignoring/removing files from project tree

When working with generic projects, Qt Creator supports adding existing files and existing directories to the project (see right-click context menu, actions "Add existing files" and "Add existing directories").

When a cmake project stores external libraries in the project tree in a dedicated subdir (for example, foo/extern), when the project is imported then all files in the extern directory is also imported. That means that the contents of the extern directory may be added to the list of project files.

Yet, sometimes including these files and directories is inconvenient and mildly annoying. Source files belonging to external projects are covered by Qt Creator's "Files in current project" list in the "Open Documents" window (ctrl+k p). Due to the nature of these files (external libs), they aren't supposed to be editable.

Apparently it's possible to remove files from the files list by editing the foo.files text file and remove each file included in the list. However, it would be much more convenient and helpful if a user could simply right-click on a file or subdirectory included in the Projects tree and run "Remove file/directory from project".