Use css also to apply line numbers colors
Updated French translation
CompletionInfo: remove weak pointer

The weak pointer was not removed (!). Hopefully this didn't cause
crashes, but Valgrind can detect the error.

Thanks to Paolo for having catched the problem.
UndoManager: more unit tests

- test group of actions (insertions/deletions between begin/end user
action).
- test modified/saved
UndoManager: improve doc
UndoManager: update_can_undo_can_redo() utility function
UndoManager: another batch of code clean-up