Change Stage getter stageVideos from notImplemented to somewhatImplemented

Babysteps for `flash.display.Stage` tracetest.
Fix various Stage accessors' behavior

Specifically, `stageWidth`, `stageHeight`, `stageVideos`, and `displayState`.

Also adds a few Stage unit tests.
Remove flash_display_Stage tracetest

All remaining tests have been integrated into the Stage unit tests.
Remove wrong description of how Graphics#_lineContainsPoint works
Fix path data deserialization bug in line hit testing

@mbebenita was, of course, right about the previous code being wrong. My thinko was that this'd affect drawing, but the deserialization code for that is entirely different.
Fix handling of aliased classes and AMF deserialization of objects with AS3 classes

Makes flash_net_classes tracetest pass.