[Frontend] fix erroneous completion across chats

When using tab-cycle nick completion mode, successfully triggering completion,
then switching to a different chat, then triggering completion again would
supply a nickname from the previous chat.

Now, if the user switches to a different chat, the completer forgets the
previous nickname list (and refuses to cycle through the completions until a
completion with different input is triggered).
Merge pull request #48 from RavuAlHemio/experiments/cycle_tab_completion2