This commit fixes the following bug:

This commit fixes the following bug:
http://bugzilla.lyx.org/show_bug.cgi?id=2900

The only drawback is that it requires about 20Mo extra-memory when loading the UserGuide. If it turns out to be too much, we can switch to a QHash based solution instead of a table.

* dimension.[Ch]:
  - Dimension(LyXFont const, char_typec): new ctor
  - set(LyXFont const & font, char_type c): new method.

* frontends/FontMetrics.h:
  - width(char_type): is now a pure virtual method.

* GuiFontMetrics:
  - CharMetrics: new structure;
  - the metrics cache now also cache ascent and descent. This is especially useful for mathed.

* MathSupport.[Ch]:
  - mathed_char_dim(): deleted. We now use Dimension::set() directly instead.

* rowpainter.C: fixe empty space.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@16124 a592a061-630c-0410-9148-cb99ea01b6c8