Allow to type a space in front of another one

Allow to type a space in front of another one

This is a follow-up to 714b731e. This fixes the issue when the cursor
is in front of a space
  abc| def
and one inserts another space (to start a word). DEPM would eat one
space and produce
  abc |def
instead of
  abc | def

Additionally, fix the same_par boolean, wheich did not take the cell
index into account.

Related to ticket #11412.

(cherry picked from commit 7848d3c90c0837e8e8549db37bf4bc5cd7e7dad1)