* new MathRow class which contains the description of a MathData
object in terms of math class and spacing
+ macros and their arguments used in the MathData object are
linearized (replaced with their contents) so that all math insets
are typeset as a string together. To this end, we introduce a
method addToMathRow to InsetMath and MathData. This method allows
to linearize recursively a MathData object.
+ It is then necessary to set manually the dimension and position of
the macros and arguments.
+ the class class and spacing are computed using the MathClass helpers.
The MathRow data is cached in the MathData object in a bufferview-dependent
way (different dpi for different screens).
* delegate most of the work MathData::metrics/draw to MathRow metrics/draw.
The case of draw is trickier, since many draw() methods rely on their
metrics without any spacing added.
