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.
Merge pull request #38 from T-J-Teru/select-after-edit-v2

Reselect `kill-ring` item after editing.