Bad TraitListEvent from ListStrEditor and TabularEditor

[mdickinson]

The ListStrEditor and TabularEditor create TraitListEvents with removed and added reversed. See:

traitsui/traitsui/qt4/list_str_editor.py

Line 335 in 3e2ce86

event = TraitListEvent(0, added, removed)

and:

traitsui/traitsui/qt4/tabular_editor.py

Line 449 in 3e2ce86

list_event = TraitListEvent(0, added, removed)

The signature of the TraitListEvent initializer is

def __init__ ( self, index = 0, removed = None, added = None ):

To avoid similar errors in the future, I recommend passing these arguments by keyword instead of by position. I think we should also make them keyword-only in Traits.

I'd mark this with "Good first issue", except that I think writing the tests may be nontrivial.

[mdickinson]

Note: the equivalent wx code has things in the right order, but could also be changed to pass by keyword instead of by position.