Allow adding files to project from warning message

If you open a source or header file which is not in a project yet, the following message appears:

Warning: This file is not part of any project. The code model might have issues parsing this file properly.

It would now be benefical, if the message bar had a button "Add..." to directly add the file to a currently opened project.

Currently, you have to select the correct project from the Projects panel and then search for the file in the "Add Existing Files..." dialog again.