[mediaframe applet] Support creation of widget on file drop on containment

Summary: Depends: D12287

Test Plan:
Dropping an image file (of listed MIME type) or a folder adds the Media
Frame widget to list of widgets proposed to be added for that file/folder,
selecting it then creates an instance with the given file/folder being set
as source.

Reviewers: Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D12288