Push notification improvements

[bwindels]

Given we can only show 1 notification at a time, we should provide a summary of all rooms we have received notifications from so far. So we get this:

1 message in a single room (implemented)

Alice
Hello

2 messages in a single room (implemented)

Alice
New messages

Multiple messages in up to 5 rooms (not implemented)

New messages
2 in Public Room
1 in Alice

Multiple messages in more than 5 (or less?) rooms (not implemented)

New messages
5 in Public Room
2 in Alice
1 in Bob
1 in Charly
1 in Hydrogen
and 3 more rooms

We can use the notification data to store the information from the push message to aggregate the appearance of several messages into one notification.