Buttons are not set up correctly in ui/qt4/progress_dialog.py

Issue: when clicked, "Cancel" button closes dialog window but does not return `self._user_cancelled = True` during update or calls `reject` function.

Possible solution: replace lines 147, 148:

``` python
if self.can_cancel:
buttons.connect(buttons, QtCore.SIGNAL('rejected()'), dialog, QtCore.SLOT('reject()'))
```

with

``` python
if self.can_cancel:
buttons.clicked.connect(self.reject)
```