css: reworked support for floatable components.

css: reworked support for floatable components.

What's done, roughly:

 * It's now possible to tuck the floating components out of the page
   flow similarly to .m-container-inflate. This required me to update
   the active section highlighting to handle this case correctly as well
   (and there's a lot more CSS for that now, ugh).
 * The .m-container-inflate has now padding after to make it better
   integrable into everything else.
 * OTOH, the all-around padding for .m-col-*-* elements is done only
   when they are direct children of .m-row. Otherwise, the padding is
   done only for .m-left-*, .m-right-*, .m-center-* and only on sides
   where it is needed.

Also added a few tests for functionality that was neglected before.