#161 Make disabled buttons look "more natively" on MS Windows

[Anton Kovalenko]

This patch makes disabled *buttons and labels look like
the "native" ones: with the "white shadow" below the
text, exactly as it's now done for disabled menu
entries (code is almost the same, too).

[Anton Kovalenko]

White shadow below the text for disabled buttons

[Jeffrey Hobbs]

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Is this code accurate when you chose alternate fg/bg colors
for the button?

[Anton Kovalenko]

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I won't dare to say it's accurate; the color of 3D highlight
doesn't depend on fg/bg colors. I'm not sure whether it
_must_ depend on them.
This code is similar to the change of 2003-12-26, by Mo
DeJong (for menu items). If the color must depend on fg/bg
for buttons, then it also applies to menus, which can have
alternate colors, too. If you describe the right way to
determine color of 3D highlight, I'll be able to fix it for
both menus and labels/*buttons.

[Pat Thoyts]

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This is present in 8.5.