Add empty rows to grid insets when numbers are too wide

Add empty rows to grid insets when numbers are too wide

This is done to ensure that numbering never overlaps equations. The
result can be different according to the bufferview, which is why
RowInfo::offset is now a std:map.

Remaing issues (how bad are theese ?):

- the numbering is considered too large when it overlaps the inset
  rect, whereas one could consider the width if the current row
  instead.

- previews may need to be adapted similarly to fit the whole screen width