set proper desktop file name

Summary:
if the desktop file isn't org.kde.$binname we need to set it manually so
desktops know what we are (this is imperative on wayland for icons to work)

I get the distinct feeling we should just do away with the conflicting
naming -.-

BUG: 382910
FIXED-IN: 17.12

Reviewers: #kde_applications, ltoscano

Reviewed By: ltoscano

Subscribers: ltoscano

Differential Revision: https://phabricator.kde.org/D8742