implement "jump to first unread" button

[azrdev]

since #127 there's a read marker, but especially in buffers with much activity one has to scroll a while (possibly hitting the "load more messages" link/button repeatedly), and squint hard not to miss the thin line marking "read until here"

A button, like the "scroll to bottom" one, would be helpful. In weechat TUI there's already a command /window scroll_unread (and default shortcut alt+u) for this.