fixes #249

This was causing some flickering when many messages were added.
Mostly visible on macOS.

fixes #357
fixes #335
fixes #230

The old algorithm during filtering will check every room
if it's part of the group. O(N*G)

The room ids for a group are now stored in a map for faster lookup
so the search can be completed in a single pass. O(N)

That's a fix to deal with long running sessions which will end
up taking more & more memory given enough time.

fixes #378

The notification spec handles -1 and 0 as special values:
-1 -> representation for "undef"
 0 -> infinitely
 values above 0 represent the timeout in ms.

The notification messages should disappear again and with 0, this insn't
the case. Setting it to -1 lets the notification manager decide the
timeout and displays the notification like all others.

The current notification spec says image-data should get used instead of
image_data.

fixes #305

The latter will not always return a valid pointer.

fixes #363

Add method to convert matrix content URIs to http links