Fix: the fault on parsing user_update events

When the 'user_update' events were received, the parser couldn't parse
them and threw the exceptions(System.NullReferenceException).
The cause of this issue was the the events have included null properties.

Considered the above, some numerical properties are casted
via nullable type.
Change image decoding strategy.
Merge pull request #125 from ktoku89/nullpo

Fix: the fault on parsing user_update events
Re-implement lazy image loading system.
Awaiting dispatcher action in scroll-locker animation.
Optimize timeline code.