commctrl: initial button theming support - next try

[Frank Richter]

Hi,
I've worked the suggestions made wrt into that new patch. In particular,
- subclassing is now properly done with SetWindowSubclass
- uxtheme is delay-loaded
- minor things like a correct "hot" flag and more cleanliness due the
better subclassing.

I've uploaded the patch to:
http://www.minet.uni-jena.de/~res/commctrl-themedbutton.diff
[or would have ~32k been an okay size for the list?]

Reviews are appreciated.

-f.r.