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.