theme refactoring.
* delite/themes/{{theme}}/global.less
  -- defines tags selectors + custom CSS classes for the theme
  -- imports delite/themes/common/global.less (custom CSS classes common to all themes)
* removed common_template
  -- use theme/global.less or common/global.less
* renamed css3.less ==>  mixins.less
* transparently load {{theme}}/global file of the current theme: widgets don't have anymore to explicitly ask the loader to load common theme CSS definitions.
* fix popup test on iOS: increased height to make sure buttons are visible