Fix for #647 - GUI Tabs do not switch on WX GUI Scope sink channels on OS X

With this patch, the bug is not showing anymore. It was only tested by
me on Mac OS X - not on Linux nor other platforms.
The possible regressions introduced by fix should be limited to the
notebook chooser forms only.

The 2 modifications seems to be necessary:
- The "changing" event seems to work where the "changed" does not
- when calling GetSelection, the notebook is not updated yet. So we need to get the information from the event itself (not the notebook)

Hint: If you already have a running GnuRadio installation, you can test the modifications very quickly, without re-building it:
Juts apply the patch on you git working copy, and launch "python gr-wxgui/python/wxgui/scopesink_gl.py". You should be able to click on the four buttons of the notebook : Ch1, Ch2, Trig, XY, confirm that the panels switch and are not empty.