[Plasmoidviewer] Allow passing "externalData" to the applet

This way an applet (like Icon, Folder, Notes) can be tested pre-populated with specific data:

plasmoidviewer --applet org.kde.plasma.icon /usr/share/applications/org.kde.kate.desktop
plasmoidviewer --applet org.kde.plasma.folder /usr/share
plasmoidviewer --applet org.kde.plasma.notes "Hello world"

Differential Revision: https://phabricator.kde.org/D3787