Fix bug #399 (crashes on systems with pyqt < 5.3)

Crash occurs in usertypes.py / Question class due to Python slots
being called on deleted Qt objects. This causes either a TypeError or
an AttributeError (probably depending on the state of the deleted Qt
object?).

Fixed by declaring slots in the Question object explicitly via
decorator "@pyqtSlot()".

Possible further TODOs:
- Find out whether this is a problem for slots in other objects as
  well.
- Create unittest for this bug (might me somewhat tricky, though).

qutebrowser/utils/usertypes.py

@@ -27,7 +27,7 @@ import operator
 import collections.abc
 import enum as pyenum
 
-from PyQt5.QtCore import pyqtSignal, QObject, QTimer
+from PyQt5.QtCore import pyqtSignal, pyqtSlot, QObject, QTimer
 
 from qutebrowser.utils import log, qtutils, utils
 
@@ -307,6 +307,7 @@ class Question(QObject):
             raise TypeError("Mode {} is no PromptMode member!".format(val))
         self._mode = val
 
+    @pyqtSlot()
     def done(self):
         """Must be called when the queston was answered completely."""
         self.answered.emit(self.answer)
@@ -317,11 +318,13 @@ class Question(QObject):
                 self.answered_no.emit()
         self.completed.emit()
 
+    @pyqtSlot()
     def cancel(self):
         """Cancel the question (resulting from user-input)."""
         self.cancelled.emit()
         self.completed.emit()
 
+    @pyqtSlot()
     def abort(self):
         """Abort the question."""
         self.is_aborted = True