Implement undo coalescing

Implement undo coalescing

if the undo element we want to add only changes stuff that was already
modified by the previous one on undo stack (in the same group), then
skip it. There is nothing to gain in adding it to the stack.

The typical use case is when doing a search and replace in a large
document that does many replacements in each paragraph. In this case,
the same paragraph would be stored repeatedly.

Fixes bug #12564.