avm1: Disable various features for SWFv4 and earlier

[Herschel]

The Flash 4 VM was very simple and did not have these notions:

• Prototype chain
• Scope chain
• Properties for child clips (TellTarget action used instead)
• Magic properties like _x (GetProperty action used instead)

Avoid these features for v4 and earlier SWFs.

[Toad06]

The Orisinal games get stuck on the loading screen, for instance Critical Zone.

The following example should reproduce the issue:

_level0/:abc = "ABC";
trace(_level0/:abc); // "ABC" in Flash, "undefined" in this PR
trace(_level0); // "undefined" in Flash and this PR
trace(aBc); // "ABC" in Flash, "undefined" in this PR

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Also, a strange thing is that the dot notation can usually be used in Flash Player and SWFv4...

o = {};
o.def = "DEF";
trace(o.dEf); // "DEF" in Flash and this PR

... but it won't work with _levelN:

_level0.ghi = "GHI";
trace(_level0.ghi); // "undefined" in Flash and this PR
trace(ghi); // (same thing)

Not sure if that really matters though.

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Sample (SWF)