Support multiple values in XDG_CURRENT_DESKTOP

Summary:
The Desktop Entry spec says "If $XDG_CURRENT_DESKTOP is set then it contains a colon-separated list of strings."
I found this bug (in a 2014 commit of mine) by simply reading this code and wondering why there was a QStringList.

Test Plan: Unittest updated, fails without the fix

Reviewers: apol, aacid

Reviewed By: apol

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26900