Performance improvements

More efficient database:
- Chats list query now is 5-10x faster or more in some cases. Now querying additional details only when delegate is loaded.
- Messages fetch query initially loads only the last 30 messages. The rest are loaded when scrolling half way up.
- Message changes do not cause the chats list to fully reload anymore. Instead only a single item is updated.
- Deleting entire conversations now only update a single row in the chat list rather than causing a full reload.

Improved scrolling experience:
- Asynchronous image loading reduces stutter.
- Fixed mouse propagation which caused accidental back swipe while flicking.
- Added button to quickly scroll down to most recent message. The button also shows the number of new messages that have arrived since scrolling up.

![spacebar-scroll](/uploads/82febc24933c6d58938055b720b16013/spacebar-scroll.png)