Instead of relying on a udev file running a script, poll for udev
events via libudev from a session script.  This simplifies things
nicely by avoiding to have to use sudo in the script to change to the
right user.

* configure.ac (XRANDR, SUDO): Remove command checks.
* x-resize/config.scm.in (%sudo-command, %who-command): Delete variables.
* x-resize/main.scm (drm-event-callback): New procedure.
(main): Poll for an event via guile-udev; call the above callback in
case an event is fired.
* x-resize/utils.scm (get-x-displays, get-xrandr-monitor): Delete
procedures.
(who, x-display?, sudo-command-prefix): Likewise.
(get-screen-resolutions): New procedure.
* x-resize.desktop.template: New file.
* hall.scm: Register it.
* Makefile.am (EXTRA_DIST): Likewise.  Adjust rest accordingly.
* README.org: Update doc.