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.