Every unsent tab was called "New session", so a row of them said nothing
about which was which. Name each one from its composer, using the same
first-line, word-boundary rule the backend's derive_title applies a moment
after the draft is finally sent — so the name the tab already shows is the
name it keeps.
The title moves with the composer, which is far faster than a pane
contribution should be re-registered. Panes can now render a tab label
instead of declaring one, so the label subscribes to its own key and a
rename repaints one string rather than the panes area.
Brooklyn Nicholson
c143ec7f0c