Add math cell positions to TexRow

Add math cell positions to TexRow

This is preliminary work for extending the cursor<->row tracking to math.

TexRow used to associate, to each row, a location id/pos where id determines a
paragraph and pos the position in the paragraph.

TexRow now associates to each row a list of entries, text or math. A math is a
pair uid/idx where uid will determine a math inset and idx is the number of the
cell.

The analogy id/pos<->inset/idx works better than the analogy id/pos<->idx/pos,
because what matters for the TexRow algorithm(TM) is the behaviour in terms of
line breaks.

This only improves the source view and the forward search, not the error report
and the reverse search (though this could be easily added now).