Fix fonts used by sub/superscript in text

Fix fonts used by sub/superscript in text

Several changes are needed to get things right:
* nested supersripts only work with inheritFonts=true.

* to get caret position right, it is necessary to remember the outside
  font of the inset before computing metrics.

* to get the size right at insertion time, it is necessary to trigger
  a metrics computation just after inset insertion.