Add autotest for loading applets

Nicolas Fellarequested to merge
work/nico/applet-test into master

Loading applets is increasingly complex code, with different supported formats and cases.

Add a test that creates applets and tries to load them.

Currently covers plasma_add_applet with and without a custom class. Can be extended to cover more cases (KPackage, X-Plasma-RootPath etc) in the future

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