Have all krunner plugin desktop files prefixed with "plasma-runner-"

Summary:
With all desktop files being installed in KDE_INSTALL_KSERVICES5DIR,
having a namespace prevents clashes and also helps identifying the files.

Reviewers: #plasma, broulik, apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10515