When enabled, the mouse cursor is hidden when the user types in the
terminal. It is un-hidden when the user moves the mouse, or when the
window loses keyboard focus.
* Move 'scrollback' to 'scrollback.lines'
* Move (the new) 'scrollback-indicator-*' options to
'scrollback.indicator-*'
Log a deprecation warning when the old 'scrollback' option is used,
but don't remove it, yet.