1.5 button css and demos

- [x] All: Replace "btn" by "button"
- [ ] Button: Review button CSS in core.css (compare with https://github.com/jquery/jquery-ui/blob/button/themes/base/button.css)
- [ ] Button: Merge and update Button and Button Markup demo pages
- [ ] Controlgroup: Update CSS
- [ ] Controlgroup: Update demo page
- [ ] Checkboxradio: Update CSS
- [ ] Checkboxradio: Update Checkbox and Radio buttons demo pages
- [x] Icons: Change CSS to use spans instead of pseudo elements
- [ ] Icons: Update Icons demo page
- [ ] All: Update widgets that use icons to generate spans instead of adding icon class to the element + update the "Enhanced" examples on demo pages
  - [ ] Listview
  - [ ] Collapsible
  - [ ] Selectmenu
  - [ ] Navbar
  - [ ] Toolbar (back button)
  - [ ] Textinput (clear button and search icon)
- [ ] Demos: Add spans for icons and remove icon classes from anchor/button elements
  - [x] Update custom CSS to spans instead of pseudo elements
  - [x] Header search and menu buttons
  - [x] API docs link buttons
  - [x] h2widget / quick links
  - [ ] jqm-contents.php (pre-enhanced collapsibles)
  - [ ] Check global navmenu and search
  - [ ] Check all demo pages

(Replaces PR https://github.com/jquery/jquery-mobile/pull/7753)