Update krunner plugin template

Summary:

• clean-up CMakeLists.txt from unused things
• move source files into subdirectory src/
• prepare usage of ki18n
• add license headers, default to lgpl 2.1+
• prefix desktop file with "plasma-runner-"

Reviewers: broulik

Reviewed By: broulik

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D10511