[Merge] ~ubuntustudio-dev/casper:main into casper:main

[Erich Eickmeyer]

Considering plasma.desktop no longer exists in /usr/share/xsessions in oracular onward, it shouldn't be considered unless one wants to boot into the wayland session which, in my testing, is problematic on virtual machines, namely virtualbox. The only two sessions that exist are /usr/share/xsessions/plasmax11.desktop and /usr/share/wayland-sessions/plasma.desktop. 

This could also have been accomplished with a symlink, but this method seemed to be more correct.
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https://code.launchpad.net/~ubuntustudio-dev/casper/+git/casper/+merge/473183
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