Draft: notification: Be more specific when the message is an SMS

This is about phones after all. The im category isn't in the xdg spec yet. I plan to submit this (similar to the recently added catgories for calls) but until then our best bet is to use an x-* namespace.

As im.received is still nicer than the generic notification category we only set sms.received for notification servers that are known to support it.

Depends on World/Phosh/phosh!1540 (merged)