Allow editing contentfile from a package

Filing this item separately from #603 because we want to consider them individually because of timing issues.

When a package adds a file to a project it could be desired to edit it by the user and override the file in the package

Examples:
1. The package bring a default configuration file
2. The package bring a cs file that either has a bug, or the class needs to change from public to internal.

The suggestion is that an edit operation will bring up a dialog, that tells the user that he is overriding a file coming from a package.

The user can then edit the file, and save it in place, the build task will stop including the original file from the package.