Eavesdrop on the Notify call of the org.freedesktop.Notifications
dbus-interface, proxy all caught notifications to our peer device and
track them in the internal notifications-list.
Also fix "cancel" requests from peer devices, by cutting of
kdeconnect-android's id-prefix.
The parent of the QDbusAdaptor has to be a Device, and now it was a
KdeConnectPlugin for the Notifications plugin.
Instead of passing the parent (that is actually a Device) to the
QDbusAdaptor as we were doing in the Battery plugin, I preferred to get the
actual Device from the KdeConnectPlugin, even though it needs a const_cast.
