Begin work on maintaining search history. Add to the search history upon dismissing the search bar (even on ESC, ctrl-[, etc - Vim, surprisingly, does this). We take an unorthodox approach, here, as we want to keep the history invisible unless it is specifically invoked via ctrl-p or ctrl-n - this involves *not* setting the QCompleter as the completer for m_edit. ctrl-p and ctrl-n scroll through the history, wrapping around if necessary.