Split browse-kill-ring-preview-update into two.

Split browse-kill-ring-preview-update into two,
browse-kill-ring-preview-update-text and
browse-kill-ring-preview-update-by-position, this makes the worked core
available for reuse in a later commit.  Should be no user visible
changes from this commit.
When editing `kill-ring' entries update preview.

When preview is turned on, and editing entries from the `kill-ring'
update the preview live.
Factor out find item logic from browse-kill-ring.

Factor out code from `browse-kill-ring' for finding an item in the
`kill-ring', creating `browse-kill-ring-find-entry', this will be used
in a later commit.  There should be no user visible changes from this
commit.
Reselect kill-ring entry after editing.

When editing an entry from the *Kill Ring* buffer, reselect the entry
once editing is finished, or editing is aborted.
Improve deleting of the last item in kill-ring.

When deleting the last item in the *Kill Ring* buffer, delete the
preceding separator.  The buffer display is now consistent after
deleting items from the buffer.
Merge pull request #38 from T-J-Teru/select-after-edit-v2

Reselect `kill-ring` item after editing.
Merge branch 'delete-last' of github.com:T-J-Teru/browse-kill-ring into T-J-Teru-delete-last
Merge branch 'T-J-Teru-delete-last'
Merge branch 'edit-with-preview-v2' of github.com:T-J-Teru/browse-kill-ring into T-J-Teru-edit-with-preview-v2
Merge branch 'T-J-Teru-edit-with-preview-v2'