Add QHeaderView.setSectionResize for Qt4 layer

[mottosso]

Hi qtpy,

As discussed in #65, I've got a problem, and it occurs at random.

Reproducible

# Ubuntu 16.04
$ apt-get update
$ apt-get install python3-pyside git
$ git clone https://github.com/spyder-ide/qtpy.git
$ export PYTHONPATH=$(pwd)/qtpy
$ python3
>>> from PySide import QtGui
>>> # setSectionResizeMode is added by qtpy on import
>>> assert not hasattr(QtGui.QHeaderView, "setSectionResizeMode")
>>> import sys
>>> from qtpy import QtWidgets
>>> app = QtWidgets.QApplication(sys.argv)
>>> widget = QtWidgets.QTreeView()
>>> qheaderview = widget.header()
>>> qheaderview.setSectionResizeMode(qheaderview.Fixed)
AttributeError: 'PySide.QtGui.QHeaderView' object has no attribute 'setSectionResizeMode'

The error there is thrown 9/10 times I try. :(

If I remove the line with hasattr, it works 10/10, so I figure some magic (threaded initialisation?) must be occurring on this line.

[goanpeca]

@mottosso maybe we need to override the QTreeView, QTableView classes so that the __init__ method adds the patched QHeaderView.

class QTableView(OriginalQtTableView):
    def __init__(self, *args, **kwargs):
        super(QTableView, self).__init__(*args, **kwargs)
        self.setHorizontalHeader(QHeaderView(self))
        self.setVerticalHeader(QHeaderView(self))

But if we are doing this, I would rather not use patches, but override classes as needed.

[mottosso]

Possibly.

The trouble I've found with replacing classes with subclasses, as opposed to monkey patching, is that Qt is still returning the original classes from calls such as .parent() from another widget. It might not be a problem in most cases, but is a little inconsistent and perhaps unexpected at times.