[BUTTON] Split button emulation improvements.
[BUTTON] Split button emulation improvements.

Removed misuse of BS_GROUPBOX flag for our purposes.
Implemented BM_SETSTATE, BM_GETSTATE, BM_SETSTYLE.
Handling of WM_LBUTTONDOWN simplified.
Corrected tests about button type.

mity/mctrl,mity/mctrl,mity/mctrl,mity/mctrl