remove IE6 support for icons
Move dijit.css rules into themes/common/common.less
Move claro/Common.less, Common_rtl.less rules into themes/common/common.less, dnd.less, dnd_rtl.less.
Also creating empty common_rtl.less since compile script expects it.
Refactor theme and CSS loader to support widgets that load multiple CSS files.

Changed dui/css! and dui/themes/load! to be able to specify multiple CSS files to
load as a comma separated list, and have them inserted into the document in the specified order.

Button loads CSS resources common.js, common_rtl.js, Button.js, and Button_rtl.js, and they need
to be inserted into the document in that order.  Previously they were inserted in a
random order.

The other approach I considered, but didn't implement, was to have dependencies
between CSS modules, so that Button.js starts as define(["./common.js"], ...).
multi-channel Button widget