Skip drawing of markers in non-editable math data

Skip drawing of markers in non-editable math data

There is no reason to reserve pixel space in macros replacement text,
which is not editable. This makes macros more compact and eases the
writing of lib/symbols.

* introduce new InsetMath::drawMarkers and friends that do nothing
  when nested inside a macro. This required to move macro_nesting
  inside MetricsBase, and to pass MetricsInfo & to metricsMarkers.

* keep track of nesting when drawing rows or macros.