Replace notification context properties with a DBus service

These properties are only ever used by the messaging and voicecall UIs to indicate when inboxes/callhistory/conversations are observed, to clear notifications. That doesn't require the use of contextkit or statefs.

Instead, use the existing commhistory service and have clients call methods on that directly. The service, which was unused by our stack, is also renamed from com.nokia to org.nemomobile.

Updates will be needed to nemo-qml-plugin-messages-internal and possibly a voicecall component (which should be using this if it isn't already). I will submit those PRs before merging this.