comctl32: improve subclassing

[Kevin Koltzau]

With this, Winamp 5.04's Modern theme works (mostly..some drawing glitches)
Note: Winamp 5.0's Modern theme does not work, not sure about other versions

Changelog
	Make SetWindowSubclass behave with SetWindowLong subclasses
	Allow unlimited number of subclasses
	Correct issue when SendMessage is called from within a subclass proc
	Add regression test
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