search-again in frame:searchable<%>
Searches for the text in the search edit in the result of
get-text-to-search.
( -> boolean
send a-frame:searchable search-again direction beep?)
direction = : previous searching direction'forward or 'backward
beep? = : bool#t
Returns #t if the text is found and sets the selection to the
found text. If the text is not found it returns #f.