uterm: move video API into uterm_video.h

Split the main header to avoid having it as dependency in so many places.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
uterm: move input related API to uterm_input.h

This splits off the uterm-input API from uterm.h to avoid depending on
uterm.h everywhere.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
uterm: move uterm-vt into uterm_vt.h

This splits off uterm-vt from the main header to reduce the dependencies
on uterm.h.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
uterm: move uterm.h to uterm_monitor.h

uterm.h contains only uterm-monitor related APIs so rename it.

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