Rename variables that refer to global objects.

This clarifies whether those variables will point to a Window object or an
arbitrary global object.

Note in particular that all IDL Constructors will accept an arbitrary global
object.
Simplify the implementation of cx_for_dom_reflector.
Put some 'use' statements in alphabetical order.
Make some DOM fields private.

Relevant to #2242.
Remove some Cells for immutable members.
Introduce abstractions for global scopes.

Part of #2811.
Support FormData outside Window scopes.
Add newlines in global.rs.
Merge pull request #2839 from Ms2ger/globals

Introduce abstractions for global scopes; r=Manishearth,larsberg