Update _widgets.py

[giorgioguttilla]

If you want to create a wxpython window outside of pyo's functionality, this change will allow you to do so without affecting anything else that I can find

[belangeo]

Thank you!

That opens cool possibilities like:

>>> from pyo import *
>>> s = Server().boot().start()
>>> a = SineLoop(200, 0.1, mul=0.3).out()
>>> a.ctrl()
>>> app = wx.App()
>>> a.ctrl() # Now the slider window pops up from nowhere!
>>>