allow elisa to open external files, scan them and enqueue them
Summary: add tests for enqueuing files and fix one issue
Test Plan: one can open a file by using "open with" or in a terminal by doing elisa musicFiles.ogg
Reviewers: Elisa, astippich
Reviewed By: Elisa, astippich
Maniphest Tasks: T7504
Differential Revision: https://phabricator.kde.org/D9444