Cannot tune the resolution of my external screen with Kubuntu 13.04

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http://www.kubuntuforums.net – Dear all, I have a Dell XPS13 connected via a mini-display port+DVI cable to an external monitor. I have installed Kubuntu 13.04 and all is working well, EXCEPT that the resolution on the external monitor only mirrors the resolution of my laptop screen (1920x1080). I only see one "default" screen when I try to set it up (settings-> display, or kcm_screen). The max resolution of the external monitor being higher (1920x1200 instead of 1920x1080) the bottom part of the monitor stays black. I would like to clone the screens but really get the max resolution on my external monitor. (HowTos)