Fix borders of InsetMathGrid

Fix borders of InsetMathGrid

* New virtual functions leftMargin() and rightMargin() to get rid of
  drawWithMargin()

* Factor and rewrite code for borders.

* Fix several offset calculations.

Known issues:

* Borders of multicols look too good and do not correspond to the pdf
  output. (non-regression)

* Bounding box for Hull (Regexp) not pixel-perfect.

* Bounding boxes of Diagram, XYmatrix, are too tight when there are
  borders. Also border should be disabled. (non-regression)