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 
Could you put in a comment explaining "Skip()"?

Also, why is it that we need to bind a paint event anyway? Is there something wrong with the WX backend? Should we use draw_idle()? So many questions here...