Add anew config option, ‘bell=none|set-urgency’. When set to
‘set-urgency’, the margins will be painted in red (if the window did
not have keyboard focus).
This is intended as a cheap replacement for the ‘urgency’ hint, that
doesn’t (yet) exist on Wayland.
Closes#157
This deprecates/renames scrollback-up/down to scrollback-up/down-page.
It also renames scrollback-up/down-half to
scrollback-up/down-half-page, and adds the new bindings
scrollback-up/down-line.