core: Remove main_window in favor of users creating their own windo…

[foxnne]

…ws. Added event window_open which is called when the platform has finished initializing the window

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[foxnne]

Note that this changes how the user will interact with their window. The user is free to create a new window in App.init using core.windows.new, where they will have access to all window fields. However, the window will not be finished initializing until Platform.tick runs later on. So instead of immediately doing all pipeline setup in App.init, the user will need to defer that logic until the window_open event is run.