Add/remove file to the project

At the moment we have implemented the Project/Add file to use when we want to add currently open file to the project.

This enhancement expands on that. We want to dinamycally change the menu item to either Project/Add file or Project/Remove file depending whether the file is not in the project or has been added already. This should be handled by the current page change event handler.

If at some point we want a Yes/No dialog, or something more, we will make a new ticket.