Fix display of a math hull inset in a tight inset

Fix display of a math hull inset in a tight inset

This is a kind of hack. This allows InsetMathHull to state that it
needs some elbow room beyond its width, in order to fit the numbering
and/or the left margin (with left alignment), which are outside of the
inset itself.

To this end, InsetMathHull::metrics() sets a value in
MetricsInfo::extrawidth and this value is recorded later in the
corresponding row element's `extra' field.

The code could be reorganized to be simpler, in particular by
computing metrics in tokenizeRow, or after tokenizeRow. However the
choice here is to produce a simple patch, fit for 2.4.0.

Fixes bug #12320.