Make caret visible inside math macros arguments

Make caret visible inside math macros arguments

The first step is to move the MathRow cache to BufferView, alongside
coordCache. This was on the todo list anyway, since it allows to let
go the math row information when the math equation is not on the
screen anymore. With the old scheme, it would always remain in memory.

Then, when computing caret size in MathData::metrics, make sure that
the mathrow of the elements that are linearized in the MathRow object
get their caret size information initialized too.

Fixes bug #11587.