Added a Qt4 simple example to test %gui functionality.

This is put in examples/ and not as a test because it requires manual
interaction and opens windows, but this way we can test it at least manually
during development.  Next we'll add similar ones for the other toolkits.

docs/examples/core/gui-qt.py

@@ -0,0 +1,37 b''
+#!/usr/bin/env python
+"""Simple Qt4 example to manually test event loop integration.
+
+This is meant to run tests manually in ipython as:
+
+In [5]: %gui qt
+
+In [6]: %run gui-qt.py
+
+Ref: Modified from http://zetcode.com/tutorials/pyqt4/firstprograms/
+"""
+
+import sys
+from PyQt4 import QtGui, QtCore
+
+class SimpleWindow(QtGui.QWidget):
+    def __init__(self, parent=None):
+        QtGui.QWidget.__init__(self, parent)
+
+        self.setGeometry(300, 300, 200, 80)
+        self.setWindowTitle('Hello World')
+
+        quit = QtGui.QPushButton('Close', self)
+        quit.setGeometry(10, 10, 60, 35)
+
+        self.connect(quit, QtCore.SIGNAL('clicked()'),
+                     self, QtCore.SLOT('close()'))
+
+if __name__ == '__main__':
+    app = QtCore.QCoreApplication.instance()
+    if app is None:
+        app = QtGui.QApplication([])
+
+    sw = SimpleWindow()
+    sw.show()
+
+    app.exec_()