Currently the build without implicitly included precompiled header
is not supported anyway (because Qt MOC source files do not include
stdafx.h, they include plain headers).
So when we decide to support building without implicitly included
precompiled headers we'll have to fix all the headers anyway.
Two classes base::Observable<Event> and base::Subscriber were added.
base::Observable<Event> can notify about Event-s, while any
base::Subscriber can subscribe and then async receive them.
