[Notifications] Address fall-out from Plasmoid attached property port

Globals (popup handling) is a singleton and as such has no access to context properties like plasmoid. To remain "API-compatible" there is a plasmoid property in Globals.qml.

However, this broke with the port to a singleton type and is admittedly bad layering, so I am not grumpy about this breakage! :)

• Move drag handling into a proper singleton type to avoid cross-referencing plasmoid from ten layers deep down the code.
• Request active focus from Globals.qml rather than deep down in NotificationItem code
• While at it, remove the unused clipboard code in there, too