[UXTHEME] Greatly reduce the number of times we open the theme data for the non client area. - Implement OTD_NONCLIENT for OpenThemeDataEx and OpenThemeDataFromFile. - Open the WINDOW or the SCROLLBAR theme classes only when needed. Use OpenThemeDataEx instead of the internal MSSTYLES_OpenThemeClass. Cache the open theme in the WND_DATA for later use.