Inhibit read only before deleting Kill Ring entries.

This fix is from http://bugs.debian.org/224751.  Disable all read only
properties before trying to delete an entry from the *Kill Ring*
buffer.
Delete items that are edited to the empty string.

When using the edit feature of browse-kill-ring, if an item is edited to
the empty string then it is deleted from the `kill-ring'.  Care is taken
to ensure that the correct entry is reselected after the delete in order
to give the same user experience as if the user had just chosen to
delete the item from the kill-ring with browse-kill-ring-delete.
Merge pull request #43 from T-J-Teru/inhibit-read-ony

Inhibit read only before deleting Kill Ring entries.
Merge branch 'edit-delete-empty-v4' of github.com:T-J-Teru/browse-kill-ring into T-J-Teru-edit-delete-empty-v4
Merge branch 'T-J-Teru-edit-delete-empty-v4'