add 'next unfolded' and 'previous unfolded' as thread move commands

When I open a thread in a search buffer for tag:unread, then I want to be able to cycle through all unfolded unread messages with a single button without having to search for them. This patch adds that functionality.

It also contains functions to select the next matching and previous matching but as far as I can see the ThreadBuffer does not know about the current search query?