CSS: resize and center checkboxes and radio buttons

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• make them icon-size (two subcells width and height)
• vertically align them in the text
• center them vertically in toolbars
• unify the CSS rules shared by checkboxes and radios

[manuq]

See examples:

• http://sugarlabs.github.io/sugar-web-samples/checkbox.html
• http://sugarlabs.github.io/sugar-web-samples/radio.html

[manuq]

Examples for the textarea:

• http://sugarlabs.github.io/sugar-web-samples/textarea.html
• http://sugarlabs.github.io/sugar-web-samples/textarea-expand.html