Fix Qt canvas resize_event

The Qt backend doesn't fire `resize_event` when resizing so callbacks connected to the event never get called. The following example should print when the figure is resized:

```python
import matplotlib as mpl
mpl.use('Qt4Agg')
import matplotlib.pyplot as plt

def printer(event):
    print('callback')

fig = plt.figure()
fig.canvas.mpl_connect('resize_event', printer)
plt.show()
```

This PR just adds a call to `resize_event`.  (Some slight refactoring also wandered into the commit.)