[mediaframe applet] Fix defunct adding of new files/folder per DnD

Summary:
It was broken for 2 reasons:

• usage of since-renamed MediaFrame::has() method
• calling push() directly on plasmoid.configuration.pathList was no-op due to QTBUG-67773

Test Plan:
Dropping files & folders onto an existing mediaframe widget results in the
files & folders being added now.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D12287