Use dojox/mobile/themes less infrastructure as basis for dui/themes.

Renames iphone to ios.
lightweight css loader
dui theme loader: loads CSS for specified widget for page theme
boilerplate: don't manually include CSS files; the widget will do it automatically
add example widget called ExampleWidget
dui/css! enhance to load AMD modules too
dui/themes/load: load AMD modules containing CSS

dui/themes!Button will now look for Button.js and Button_rtl.js files
(returning a string of the CSS) in the appropriate theme subdirectory.

dui/themes!Button.css will give the old behavior, loading Button.css
 and Button_rtl.css from the appropriate theme subdirectory.
wrap generated CSS files as AMD modules
update ExampleWidget to load CSS as an AMD module
Add placeholder desktop theme.
Currently based on dojox/mobile/custom, but probably will replace w/claro sometime.
ExampleWidget update (after JS conversion)
add common.js generated file to themes check in