tsm: screen: introduce scrollback-buffer line IDs

tsm: screen: introduce scrollback-buffer line IDs

If we have pointers somewhere into the scrollback-buffer, there is not
easy way to see which of two lines comes first. Therefore, we introduce
scrollback buffer IDs. These are unique and every line gets one assigned
when it is linked into the sb-buffer. These IDs guarantee that front lines
have lower IDs than bottom lines.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>