Use weakref for tabs in QShortCut lambdas

Unfortunately we have no way to directly destroy or remove the
lambdas embedded in the QShortcut objects, so this is the
only solution to avoid leaking references.  As the QShortcut
objects have the window as parent, they are destroyed with the
window so dangling refs to the destroyed window can't happen.

This and 91349d109e fix #1549.

gui/qt/main_window.py

@@ -20,6 +20,7 @@ import sys, time, threading
 import os.path, json, traceback
 import shutil
 import socket
+import weakref
 import webbrowser
 import csv
 from decimal import Decimal
@@ -144,14 +145,15 @@ class ElectrumWindow(QMainWindow, PrintError):
         self.setWindowIcon(QIcon(":icons/electrum.png"))
         self.init_menubar()
 
+        wrtabs = weakref.proxy(tabs)
         QShortcut(QKeySequence("Ctrl+W"), self, self.close)
         QShortcut(QKeySequence("Ctrl+Q"), self, self.close)
         QShortcut(QKeySequence("Ctrl+R"), self, self.update_wallet)
-        QShortcut(QKeySequence("Ctrl+PgUp"), self, lambda: tabs.setCurrentIndex( (tabs.currentIndex() - 1 )%tabs.count() ))
-        QShortcut(QKeySequence("Ctrl+PgDown"), self, lambda: tabs.setCurrentIndex( (tabs.currentIndex() + 1 )%tabs.count() ))
+        QShortcut(QKeySequence("Ctrl+PgUp"), self, lambda: wrtabs.setCurrentIndex((wrtabs.currentIndex() - 1)%wrtabs.count()))
+        QShortcut(QKeySequence("Ctrl+PgDown"), self, lambda: wrtabs.setCurrentIndex((wrtabs.currentIndex() + 1)%wrtabs.count()))
 
-        for i in range(tabs.count()):
-            QShortcut(QKeySequence("Alt+" + str(i + 1)), self, lambda i=i: tabs.setCurrentIndex(i))
+        for i in range(wrtabs.count()):
+            QShortcut(QKeySequence("Alt+" + str(i + 1)), self, lambda i=i: wrtabs.setCurrentIndex(i))
 
         self.connect(self, QtCore.SIGNAL('payment_request_ok'), self.payment_request_ok)
         self.connect(self, QtCore.SIGNAL('payment_request_error'), self.payment_request_error)