uterm: new video backend

uterm: new video backend

Our old backend was hacked together and hadn't have any structure. This is
a new approach to create the uterm-library inside kmscon. The
uterm-library will contain everything that is needed to run an application
on Linux DRM devices without X11 or Wayland support.

The idea is to move the input subsystem to uterm, too. No other stuff is
currently planned to be included in uterm.

Although uterm is supposed to be a separate library, we do not build it as
such library. We currently include the log-subsystem and the
eloop-handlers in the library so we cannot build it as stand-alone
library. However, we try to keep it separate so if we ever need to export
it, then it should be a one-hour job to do it so.

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