Refresh checks and radios

• use a dedicated mixin to style checks and radios in _drawing.scss
• still use gradients and shadows but drop the heavy accentuated borders and glyphs
• keep all special cases for checks and radios in _common.scss
• remove treeview disabled checks/radio styling because it seems no longer needed after testing with the new mixin
• add a fix for checked and indeterminate checks/radios in menus and slightly darken their borders similar to $selected_borders_color
• fix selected treeview checks in backdrop being gray

@jimmac please see if you still like it :)

Discussion can be found here: Teams/Design/os-mockups#25 (closed)