Make BrowserContext::create_window_proxy private and make it set the window_proxy field itself.
Include modules with only callbacks in BindingDeclarations.

This commit also includes improvements to the 'use' order in some of the
touched files.
auto merge of #2533 : Ms2ger/servo/callback-bindingdeclarations, r=jdm

This commit also includes improvements to the 'use' order in some of the
touched files.
auto merge of #2532 : Ms2ger/servo/create_window_proxy, r=jdm