Add "Export to xorg.conf" action

ARandR seems to favor saving screen configuration as executable scripts calling xrandr. However, that's not the only way of setting XRandR configuration, it can also be saved directly in Xorg's configuration (xorg.conf or xorg.conf.d/*.conf). For example, see this Archlinux wiki section.

I much prefer setting configuration that way, as config files, instead of scripts that I'll have to figure out how to auto-execute. I don't deny the advantages of scripts for certain use-cases such as for a laptop having to switch between different confs, but for a desktop with a configuration that never changes, IMO Xorg conf is the best way to set & forget.

So, I've implemented a menu action to "export" the configuration in Xorg.conf format. The implementation may be a bit naive or proof-of-conceptish, especially since I've never worked with PyGTK before.