Keep the almost sorted list around and sort by the raw timestamp value
instead of doing the expensive toMSecSinceEpoch conversion.

This also removes an ugly hack, that was needed before.

When user is mentioned (via matrix 'highlight_count'), inactive
rooms will use a different color for the notification circle than
when only general unread messages exist.

fixes #313

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)

- Patch the olm repo with a CMakeLists.txt file

fixes #291