overlay: Auto disconnect when the overlay widget is deleted

Register `__on_destroyed` as a Qt slot so it can be automatically
disconnected when the overlay widget is deleted/destroyed.
This fixes a possible 'RuntimeError C/C++ object was deleted ...' error
when the overlay is deleted before the target widget.

Orange/widgets/utils/overlay.py

@@ -18,7 +18,7 @@
     QWidget, QPixmap, QStyleOption, QPainter
 )
 from PyQt4.QtCore import Qt, QSize, QRect, QPoint, QEvent, QTimer
-from PyQt4.QtCore import pyqtSignal as Signal
+from PyQt4.QtCore import pyqtSignal as Signal, pyqtSlot as Slot
 
 
 class OverlayWidget(QWidget):
@@ -172,6 +172,7 @@ def getsize(hint, minimum, maximum, policy):
         geom = QRect(QPoint(x, y), size)
         self.setGeometry(geom)
 
+    @Slot()
     def __on_destroyed(self):
         self.__widget = None
         if self.isVisible():