Minor cleanup in SelectionList methods
Remove per lines timestamp in Buffer
Remove duplicated logic and fix insert in Replace mode
Add initial (and probably buggy) compute_modifications code

compute_modifications compiles a list of buffer change into
a list of Modifications that can be used for updating BufferCoord
update Modification for WordDB
Use Modification for region highlighter
Remove line modification, replaced by modification
Fix, cleanup and refactor compute_modifications
Clamp modified coord in Selection as it could be buffer end
Fix selection update in when doing a 'replace' insert
Fix Modification
Still more fixes for Modification
Avoid buffer end line in word_db update
Remove use of 'offset' in buffer iterators, lines are just String

No need to maintain line offsets anymore.
Fix segfault in region highlighter