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.
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'