[GSoC] Support of pluggable theming engines

[Ivan Akulinchev]

> Having properly implemented user controls is far more important than
> having partially working gtk3 disguise.

One thing depends on another. When I worked on my own UxTheme
implementation, I found that comctl32 likes to send 0 instead of
properly arguments. It angered me.

So if I want to get a perfect GTK3-theming, I have to fix comctl32 too.