Fix VCards and avatars

VCards no longer crash the app and are being loaded properly. Ignore
empty avatars. Cache avatars properly.
Fix #20

All messages are now saved in database properly.
Fix popup spam

Not the best solution, but doesn't matter, IT WORKZ.
Improve stuff

Increase the delay. Fix chat icon not being cleaned when closing app
using menu.
Reintroduce Message List Model

Now both the old implementation and new is working side-by-side and it's
perfect.
New chats open instantly.
Switching between chats in chats view is instant as well.
Reduce time of loading a chat with messages.
Displays only messages from current session, older are available in
archive view (I can add an option to load them if anyone cares).
No UI lag when receiving messages.

It. Feels. AWESOME!
Handle properly receiving messages

Fixes receiving messages from contacts not on contact list and receiving
them before roster is initialized (useful for GTalk). Also chat lists
now handles contact renaming.
Handle 0 accounts situation

Now Lightbulb can handle a situation when there is no account
configured.
Fix a typo and splitscreen input
Fix crash when changing contacts name

Fixes crashes when contact name is being changed but it's not on chats
list. Thank you Conrado967!
Also fix loading contact names from received roster if contact is
available on it (receiving messages before roster initialization
thingy).
Introduce contributors dialog
Handle editing accounts

Too bad removing and adding accounts crashes the app.
Try to fix crashes

But things are still crashing a lot in account management code.
On the other hand, loading avatars don't require restarting the app, but
causes a huge lag while loading them initially. You should add a dialog
later to make user disable displaying them on initial connection.