[Bugfix] qgo crash if the last path is removed

qgo crash if the last path (SGF edit) is removed, and don't start (just Segmentation fault). Then this is a fix :)
Use Qt's open file dialog with preview widget

Since the preview widget is added to the standard dialog's layout,
it might not be displayed properly on other platforms. There used to
be a good way of dealing with this problem in Qt 3, but it does not
seem that it will be reimplemented in the future. Possible workaround:
always use standard (non-native) dialogs.