Grid: Added margin for selectmenu and checkboxradio in grids. Move rule that unsets button margin to make sure the override always works.
Demos: Updated button in grid demo. Blocks should always be direct child of grid.
Listview: Add icon position class again.

It was removed to reduce adding classes to minimum, but the downside is that we had to use slow attribute selectors in the CSS. We probably also want to support left position in a future version and might need it for dir="rtl" support so I reverted this change.
Grid: Minor change in selector.
Theme: Improved the icon CSS. Only pseudo element when there actually is an icon.