Add package-files-are-editable dev flag

@albualexandru   

This is to facilitate debugging of package-related issues: sometimes one needs to play around with a package's sources to figure out something.

One thing that doesn't work currently: if a package file is open while the flag is off, and then the flag is turned on and CDE is restarted, the reloaded source is still read-only. It needs to be closed/reopened to make it writable. This is a developer-facing functionality, so it's ok for now.