uxtheme: drawing functions

[Kevin Koltzau]

With this some apps may start looking partly themed (eg. the toolbar buttons 
in newer versions of mIRC are themed), and we can start adding theming code 
to widgets

Some themes use 32bit bitmaps with an alpha channel, and AlphaBlend isn't 
implemented yet..so some images dont look quite right at the moment

Changelog
	Implement DrawThemeBackgroundEx, DrawThemeText, GetThemeTextExtent