This gets a lot of unecessary includes out of Widget.cpp. Doing this
didn't work before, but improvements in the C library and using dynamic
libraries have likely un-broken it :^).
Also, move the registration global object to an anonymous namespace. No
reason it has to be an extern symbol.
You can now #include <AK/Forward.h> to get most of the AK types as
forward declarations.
Header dependency explosion is one of the main contributors to compile
times at the moment, so this is a step towards smaller include graphs.
