Page: add dialog extension
Page: add extension to replace dialog widget
Page: add enhanced option for page and tests for dialog extension
Dialog Extension: remove call to _enhanced is now called by page
Selectmenu: Don't use shorthand for unsetting the border because it makes it harder to override.
Demos: Added custom CSS for filter inside custom selectmenu demo.
Demos: Filterable inside custom select demo custom dialog style.
Moved CSS for button elements to core.css because it's not part of button widget.
buttonMarkup requires core CSS, not the button widget CSS.
Added forms prefix to button widget CSS file.
Forgot to rename button.css in structure.css file.