Trying to solve bug-2452, I optimized some of the most significant problems outlined...

Trying to solve bug-2452, I optimized some of the most significant problems outlined in the profile report attached in there.

This commit avoids unnecessary metrics recalculations by caching the last LyXFont used. I had to cleanup the width(), ascent() and descend() redundancies by transferring that to InsetBase.

InsetMathDim should go now as it is not really needed.

* InsetBase: properly handle inset Dimension.

* InsetOld: get rid of redundant width(), ascent() and descent()

* InsetMathDim: ditto

* InsetMathChar::metrics(): avoid metrics recalculation if font is unchanged.

* InsetMathSymbol::metrics(): ditto

* InsetMathKern: use InsetBase::dim_ instead of local ones.

* InsetMathSpace:
  - use InsetBase::dim_ instead of local ones.
  - space to width transformation transferred to anonymous namespace.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@17899 a592a061-630c-0410-9148-cb99ea01b6c8