grep: Allow making a new search when one is already running

Currently if one makes a search that will definitely take ages to complete
([:alnum:] regex is a good example), then there's no way of launching a more
efficient or better search to replace the previous one. The current behavior
makes the "Find" button insensitive while a search is running, but even that
part is broken as typing new characters in the entry will trigger gbp_grep_panel_find_entry_text_changed_cb()
and make the button sensitive again…

So instead, just make it simpler and allow making searches even when there's
already one running, cancelling the previous one.
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