Permit sweep of private keys for watch-only wallet

Fixes #1585
Warn user if watching-only.
Introduce new helper class WindowModalDialog and use it in a couple
of places.  This is better than process-modal dialogs for our
multi-window daemon setup.

gui/qt/util.py

@@ -192,6 +192,14 @@ class CancelButton(QPushButton):
         QPushButton.__init__(self, label or _("Cancel"))
         self.clicked.connect(dialog.reject)
 
+class WindowModalDialog(QDialog):
+    '''Handy wrapper; window modal dialogs are better for our multi-window
+    daemon model as other wallet windows can still be accessed.'''
+    def __init__(self, parent, title=None):
+        QDialog.__init__(self, parent)
+        self.setWindowModality(Qt.WindowModal)
+        if title:
+            self.setWindowTitle(title)
 
 def line_dialog(parent, title, label, ok_label, default=None):
     dialog = QDialog(parent)