Add document.location (WIP)

[niutech]

This is a first draft of document.location support (issue #1068). It is not tested nor complete yet. Could anybody please pick up the changeset and test it? It is hard for me to understand the Worker-DOM internals, how the transfer of complex objects such as Location works between main thread and worker.

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[niutech]

I don't know why I have errors like: src/main-thread/commands/object-creation.ts(69,47): error TS2304: Cannot find name 'RenderableElement'. even thought I didn't change this file.