Trigger push notifications for other data than messages

[guusdk]

Currently, push notifications are only sent when processing message stanzas that have a non-empty body. This is not covering all use-cases where it's desirable to have a push notification. For example, when a device is the intended target of an audio/video call, then a push notification is desirable.

Prosody and eJabberd follow the business rules summed up here: https://hg.prosody.im/prosody-modules/file/tip/mod_cloud_notify/business_rules.markdown

Openfire should take inspiration from these business rules.