undoing to previous save

Sometimes Ctrl+Z undoes more than what was written since the previous time the file was saved. Steps to reproduce:

1. Make new file (Ctrl+N)
2. Write asd
3. Save file
4. Write lol
5. Undo (Ctrl+Z)

Expected result: delete only lol.

Actual result: the entire asdlol is deleted.

As mentioned below, the edit_separator() method should probably be called when saving the file.

The relevant code is likely class FileTab in porcupine/tabs.py. There is already a virtual event named <<AfterSave>> , and presumably the edit_separator() call should go into the same place where <<AfterSave>> is emitted.

Similar to #431

I would totally use this but have no idea how to implement it

See edit_separator.