mainwindow: Actually use DAR (not PAR) to set the viewer's display aspect ratio

Using the pixel aspect ratio to set the viewer's (display) aspect ratio
made no sense. We had a nice unused method for that exact purpose.

This also happens to fix bug #722003

pitivi/mainwindow.py

@@ -1047,7 +1047,7 @@ class PitiviMainWindow(Gtk.Window, Loggable):
         """
         When the project setting change, we reset the viewer aspect ratio
         """
-        self.viewer.setDisplayAspectRatio(project.aspect_ratio)
+        self.viewer.setDisplayAspectRatio(project.getDAR())
         self.viewer.timecode_entry.setFramerate(project.videorate)
 
     def _sourceListMissingPluginsCb(self, unused_project, unused_uri, unused_factory,

pitivi/project.py

@@ -711,12 +711,6 @@ class Project(Loggable, GES.Project):
         if self.set_video_restriction_value("pixel-aspect-ratio", value):
             self._emitChange("rendering-settings-changed", "pixel-aspect-ratio", value)
 
-    @property
-    def aspect_ratio(self):
-        videopar = self.videopar
-        par = videopar.num / videopar.denom
-        return par * float(self.videowidth) / float(self.videoheight)
-
     @property
     def audiochannels(self):
         return self.audio_profile.get_restriction()[0]["channels"]