When inserting math inset over selection, place cursor better

When inserting math inset over selection, place cursor better

This is a follow up to 503c7c16.

The new argument for placing cursor after insertion of inset is:
* if inset has no cell, do nothing
* otherwise, place inset in entry cell.
  + if entry cell is not empty (we pasted a selection), go to next cell
  + if this next cell does not exit, stay after the inset.