Tab index bug fix

[See line 137 of the split_editor_area_pane and line 245 of the unit test file for the important changes.]

When a tab is created with the splitEditorAreaPane, the index variable in the `add_editor` method was wrong only for the first editor. That led to the tooltip for that editor being lost. This is a hack to fix that. The real fix is probably down inside qt or pyside...

Thanks to @corranwebster for his help tracking this one!
This is the only actual change.