make wx backends compatible with wxPython-Phoenix

This is based on the comments made to my previous PR for this.

Note that wxPython Phoenix snapshots can now be installed with pip:
pip install -U --pre -f http://wxpython.org/Phoenix/snapshot-builds/ wxPython_Phoenix 
On 12/5/2014 16:40, Benjamin Root wrote:
>
> In examples/user_interfaces/embedding_in_wx2.py:
>
> > @@ -52,31 +60,20 @@ def __init__(self):
> >      def add_toolbar(self):
> >          self.toolbar = NavigationToolbar2Wx(self.canvas)
> >          self.toolbar.Realize()
> > -        if wx.Platform == '__WXMAC__':
>
> By removing these checks, are you saying that the bug they work around 
> are no longer in effect for wx v2.8 and above?
>
I don't have a MAC, I removed that based on the comment by jenshnielsen 
on Aug 29 to this PR.

Werner