Remove align centre which was causing problems

[corranwebster]

Fixes #828.

It is possible we may need to do some further tweaking of Wx layout with this being removed, but this fixes the immediately reported problem.

[kitchoi]

Confirmed this resolves some of the test failures we already saw.
This is the smaller test that fails and is fixed by this PR:

class MultipleText(HasTraits):
    """ An object with multiple traits to test layout and alignments."""

    txt1 = Str("text1")
    txt2 = Str("text2")
    txt3 = Str("text3")


@skip_if_null
class TestOrientation(unittest.TestCase):

    def test_horizontal_layout(self):
        model = MultipleText()
        view = View(
            HGroup(
                Item("txt1"),
                Item("txt2"),
                Item("txt3"),
            )
        )
        ui.edit_traits(view=view) # UI needs closing 

[kitchoi]

LGTM.

[jmoraleda]

@corranwebster Thank you for your patch. With respect to your comment it is possible we may need to do some further tweaking of Wx layout with this being removed, This (and removals of other alignment flags that trigger the same error, if you encounter them elsewhere) will have no impact in your layout because the error is caused because wx now treats as errors alignment flags that were being ignored in earlier versions of the toolkit.

The reason the culprit alignment flags are flagged is that they represent a conceptual error because the space given to the widget by the sizer along the given dimension, vertical or horizontal, is tight due to the semantics of the specific sizer being used, so there is no room for any further alignment.