Fix crash in InsetInfo when bindings change

Fix crash in InsetInfo when bindings change

This is actually a generic InsetInfo issue:

1/ the contents of the inset is computed in updateBuffer, so that it
   is available for drawing but also for latex output (think batch
   export). When it is called, it deletes the existing inner paragraph
   and replaces it by a new one

2/ metrics build a new Row object that represents to paragraph

3/ draw() relies on this information

Now, imagine that updateBuffer() is called after metrics(). This can
happen for many reasons, and does happen here (display a shortcut info
and change the shortcut file from cua to emacs).

This problem has been here forever, but is only visible now that the
(experimental) bookmark display code needs to read the underlying
paragraph id.

The solution is to compute the inset contents at metrics time. This
is done by moving the relevant code to a new standalone build() method
that is called in metrics() but also in latex().