Make sure inset buffer is correctly set in math grid

Make sure inset buffer is correctly set in math grid

Make MathData::setBuffer set the buffer of insets that it contains.
Remove corresponding code from InsetMathNest.

update the buffer() property in the following tabular-feature
actions : copy-row, add-row, copy-col, add-col.

(cherry picked from commit 837bcbb0433316ebe528b71e7c9952638ac1baee)