Add guiEvent data to Qt backend

[blink1073]

Boy do I wish I stared contributing 3 years ago...
I could have prevented detours to Chaco, PyQtGraph, guiqwt, ...ugh.

[pelson]

Thanks @blink1073 - we wish you'd started contributing 3 years ago too!
So, can you give us a heads up as to the reason for this change? This is slightly awkward in that this isn't the interface defined in backend bases..

[tacaswell]

It looks like gtk, gtk3, tk, and wx do this already.

I think the idea is that this is a way to transparently pass the gui-framework specific event objects through to your call backs.

[blink1073]

What Tom said. 😄

[blink1073]

See also: https://github.com/matplotlib/matplotlib/blob/master/lib/matplotlib/backend_bases.py#L1316

[pelson]

Cool - I learnt something new. 😄