displayconfig-gtk should detect and manage TwinView

Displayconfig should be able to detect when TwinView is in use on an "nvidia" card.

For example:
---

Section "Screen"
    Identifier "Screen0"
    Device "Videocard0"
    Monitor "Monitor0"
    DefaultDepth 24
    Option "AddARGBGLXVisuals" "True"
    Option "TwinView" "1"
    Option "metamodes" "DFP-0: 1680x1050_60.00 +0+0, DFP-1: 1280x1024_75 +1680+0; DFP-0: 1280x1024 +0+0; DFP-0: 1024x768 +0+0; DFP-0: 800x600 +0+0; DFP-0: 640x480 +0+0"
    SubSection "Display"
        Depth 24
    EndSubSection
EndSection

Section "Screen"
    Identifier "Screen1"
    Device "Videocard1"
    Monitor "Monitor1"
    DefaultDepth 24
    Option "TwinView" "0"
    Option "metamodes" "DFP-1: 1280x1024_75 +0+0"
    Option "AddARGBGLXVisuals" "True"
    SubSection "Display"
        Depth 24
    EndSubSection
EndSection
---

If TwinView is "1" on the first screen, I guess this is enough. At my current state I have a 2960x1050 desktop from these settings.

Displayconfig shows me NEC 20WMGX2@2960x1050 and VP930b@640x480.

Under "Secondary screen" I believe there should be three options:
- Xinerama (I think, someone help me with this one: appears to be spanned, but really two screens/has some complications with other software)
- TwinView (nvidia only?: weld two screens to one, generally reliable)
- Mirror

It should be able to detect all these configurations thru Xorg.conf.

- Xinerama can be detected through:

Section "ServerFlags"
    Option "Xinerama" "1"
EndSection

- TwinView by "TwinView" "1" under a screen.

- Mirror (how it currently is?)