Still show transparent window even if IsTransparentBackgroundSupported()
returns false, just don't make their background transparent and only use
SetTransparent() in this case. In fact, don't try to use transparent
background style at all by default and add a separate menu item to do it
if wanted.
Also add a slider to allow changing opacity of the window.
Finally, make the window big enough in high DPI under MSW by using
FromDIP().
279af795ed
