BUG : added failing test for passing dates to stem
BUG : minimal fix for date x-values in stem plots

closes #2602
Implement backend for PyQt5 + modify Qt4 backend to Qt5 structure.

A backend for PyQt5 based on intial work by @badders, modified to fix Qt5 mouse event handling,
then re-structured to implement as a wrapper over the existing Qt4 code. Following discussions on
this pull request:

https://github.com/matplotlib/matplotlib/pull/2471

The code has been restructured to implement PyQt5 backend as a first-class implementation, with
other Qt backends (PyQt4, PyQt4v2, PySide (Qt v4)) wrapping it and modifying as required. The issues
of objects being moved around in the Qt namespace (many QtGui objects now in QtWidgets) QtWidgets is
simply assigned as a copy of QtGui if not available. This achieves the intended outcome with the
minimum code.

PySide required re-ordering of import on FigureCanvasQTAgg or paintEvent
function would not being called on FigureCanvasQTAggBase resulting in
black window.

A number of indentation, import and other fixes.