D7561: Avoid absolute paths to .desktop files in launcher URLs.

[Eike Hein]

hein added reviewers: broulik, davidedmundson.

REPOSITORY
  R120 Plasma Workspace

REVISION DETAIL
  https://phabricator.kde.org/D7561

To: hein, broulik, davidedmundson
Cc: plasma-devel, ZrenBot, progwolff, lesliezhai, ali-mohamed, jensreuterberg, 
abetts, sebas, apol, mart, lukas