Draw display math numbering outside of inset

Draw display math numbering outside of inset

This is the first (easiest) step in fixing bugs 10668 and 11333.

The numbering is now drawn outside of the insets, which solves the
alignment problems and make editing easier.

What does not work yet:

- long labels will overwrite equations. To fix this, we need to
  implement the same algorithm as LaTeX and put labels on their own
  row when required.

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