made user-defined widths be max full width on smaller screens (except percentages)

Attention: A side effect will make most existing boxes a bit smaller, so widths
might need to be adjusted.
Updated example syntax page

* documented additional div needed for width keyword columns
* use width keyword for columns explaining columns
* moved clear float class to misc section
* switched all emulated headlines to real ones
* added explanation about difference between widths and width keywords
* clarified browser versions when non-modern browsers were mentioned
changed column toolbar picker to add a whole row of half columns

Attention: This also changed the 'column' lang string from singular to
plural. And it added 'group' to the lists of non-prefixed classes.