Use host writable statusline (VT feature) to show monomux state to the user

[christianparpart]

See contour-terminal/contour#687 for some general insights.

I'd like monomux to make use of the host writable status line. This is currently supported by recent(!) xterm as well as Contour.

You can detect support by the terminal using DECQRM.

Information I'd like to see (up to your imagination):

• What session am I connected to?
• if multiple sessions are available, maybe list the others (by index or short title or so)

Given the last bullet point, it might make sense to be able to rename sessions (just like in tmux :D)

EDIT: forgot to mention: if anything is missing in Contour, please ping back to me.