Add a MsmApp::skillsPath method so the skills folder is not hardcoded

Summary:
The skills folder was hardcoded to /opt/mycroft/skills in MsmView.qml.
When using the openSUSE packages I'm preparing, that's changed
to ~/.mycroft/skills, so now we use that one if it exists and the
system folder in other case.

Reviewers: Aiix

Reviewed By: Aiix

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D10755