Compile CSS with lessc, was not matching LESS
Add margin to the canvas

A margin of one subcell is a better default than zero.  Of course
activities will likely override the rule.
Remove harcoded numbers

Make all measures relative to the subcell size and the line width.

- an input text has a width of 3 cells
- big radius are 2 subcells
- small radius are 1/2 subcells
- small buttons have width and height of 2 subcells