Possibility to remove files from the External Resources

[yanne]

Originally submitted to Google Code by @mikahanninen on 10 Sep 2012

If something was added by mistake into External Resources it can't be removed without RIDE restart.

Implement 'remove' action which removes file from the RIDE, but leaves it intact in the file system.

The same applies into other parts of tree dialog also. Maybe it would be good to have an option to remove file from the project (ie. having resource/test case files in the directory without them being in the RIDE).