For bug #1710.  Change the ID type of mime data for filterbox drop event...

...s to Referenceable rather than using the ID's native type.

This PR does what was described in the bug report, but should be reviewed for two things:

1. Is this (i.e. change the id type to referenceable) what is wanted in all cases of drag&drop to a filterbox?
2. Is the fix in the correct class/place?