Inherit from wxCompositeWindowSettersOnly<> to make sure all the usual
setters, such as SetForegroundColour() and SetFont(), called on
wxStaticBox are propagated to the label window too.
However also prevent SetBackgroundColour() from being propagated
unnecessarily -- because the checkbox already inherits the parent
background colour by default in wxMSW anyhow -- and still override
SetFont() to adjust the label window position after the font change,
otherwise it could be truncated after increasing the font size, for
example.
Because of these issues, wxCompositeWindowSettersOnly is not ideally
suited for its use here, but on balance it still seems to be better to
use it rather than reimplement parts of its functionality here.
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