Invalidate the flow layout data on a layout request.

This makes sure that the size hints cached by the layout items are
refreshed on the next layout.

This also removes the hack workaround for setting the layout widget.
The need for the hack was removed with the previous commit which
fixed a bug in QSingleWidgetLayout.

enaml/qt/qt_flow_item.py

@@ -2,7 +2,7 @@
 #  Copyright (c) 2012, Enthought, Inc.
 #  All rights reserved.
 #------------------------------------------------------------------------------
-from .qt.QtCore import QSize
+from .qt.QtCore import QSize, QEvent
 from .qt.QtGui import QFrame, QLayout
 from .q_flow_layout import QFlowLayout, AbstractFlowWidget, FlowLayoutData
 from .q_single_widget_layout import QSingleWidgetLayout
@@ -182,16 +182,19 @@ def setFlowWidget(self, widget):
 
         """
         self._flow_widget = widget
-        try:
-            self.layout().setWidget(widget)
-        except AttributeError:
-            # XXX Hack! The layout type gets swapped out from under us
-            # during the destruction process when running on PyQt. It
-            # comes out as an instance of QWidgetItem. Need look into
-            # exactly what is causing this.
-            l = self.layout()
-            if widget is not None or isinstance(l, QSingleWidgetLayout):
-                raise
+        self.layout().setWidget(widget)
+
+    def event(self, event):
+        """ A custom event handler which handles LayoutRequest events.
+
+        When a LayoutRequest event is posted to this widget, it will
+        emit the `layoutRequested` signal. This allows an external
+        consumer of this widget to update their external layout.
+
+        """
+        if event.type() == QEvent.LayoutRequest:
+            self._layout_data.dirty = True
+        return super(QFlowItem, self).event(event)
 
 
 AbstractFlowWidget.register(QFlowItem)