Fix issue with change-tracked deleted display math

Fix issue with change-tracked deleted display math

- If a display math not starting a new paragraph is deleted, the
  current \lyxdeleted macro (if any) must be closed and a new one
  started, otherwise the display math will be shifted up.

- Use \linewidth instead of \columnwidth because the former will adapt
  to the reduced horizontal width in list environments, avoiding shifting
  to the right the diplay math.