Remove InsetMathSqrt hack from MathMacro

Remove InsetMathSqrt hack from MathMacro

The expanded cells of a mathmacro were previously stored in an InsetMathSqrt.
This was only used as a container for the MathData object in the first cell
of the sqrt inset, which contained the actual expanded arguments.
Funny enough, the only place were the inset property of expanded_ was really
used cannot be seen in the diff. It was MathMacro::kerning(), and this usage
was wrong, since InsetMathSqrt::kerning() always returns 0. Threfore, using
the correct type (MathData) for expanded_ does not only make the code more
readable, gets rid of an unneeded dependency, but also fixes a bug: Now the
correct kerning is returned for expanded cells. Also, expanded_ and
definition_ use the same type now, which looks nicely symmetric.