can't drag'n'drop onto an unsaved file

This bug has been split from https://bugs.launchpad.net/scratch/+bug/1397103.

When a file is deleted while being edited, an infobar is shown and the SourceView is set to non-editable. This leads to Scratch rejecting the open file drag'n'drop interaction.
To reproduce:

1. open any file
2. delete or rename that file so the yellow "file missing" infobar shows
3. try to drag'n'drop a file into Scratch
   the file will be rejected and revert

The problem originates in the code that shows the infobar and sets the SourceView to read-only. Gtk won't allow the reception of drag'n'drop events on a read-only object. Reading around on the web, one needs to listen for the drag-start hook and set the object to read-write, then when the drag leaves or a file is dropped, set it back to read-only. This will be a pretty complicated job.