Comment 3 for bug 1852009

It seems neatest to do it as a termprop; that way applications can be informed when its value changes (in case they need to hook up extra event handlers, etc...)

I'll take the first one, though renamed `VTERM_PROP_FOCUSREPORT` since none of the other props (e.g. cursor-related ones) have additional underscores in parts of the names.

It also needs some tests, which I shall add.