IVE: Function deletion during title editing

Sometimes the Delete button will delete the complete function while editing the text (instead of deleting a letter) That happens especially after actually deleting the symbol then doing undo and then editing the text

Worse: I just got a segfault after doing Ctrl-Z after deleting a function

I also had a case where I did Ctrl-Y (or Ctrl-Shift-Z, to Redo) and the name of the function was reset to Function 1 (the default name)