Better mechanism for setting cell cell height

Better mechanism for setting cell cell height

When computing a cell metrics, it is now possible to specify whether it
is tight (at least as tall as 'x') or not (as tall as the max height of
the font).

Use this to make sure that grid insets have large enough cells. It
will probably appear that other cells needn't be tight. Currently, the
only cell which is known to be tight is the nucleus of the root inset.
Others should be examined one by one. It might be that the default of
MathData::metrics tight parameter should be `false'.

Fixes bug #11050.