viewer: provide UI to set preview resolution

Previews don't need to be rendered full resolution, particularly since the
preview is hardly ever 1:1 size of the sequence. The functionality to render
at lower resolutions already existed, but there was no UI for it. This commit
implements UI to set the resolution divider on the viewer.

app/config/config.cpp

@@ -73,6 +73,7 @@ void Config::SetDefaults()
   config_map_["AddDefaultEffectsToClips"] = true;
   config_map_["AutoscaleByDefault"] = false;
   config_map_["Autoscroll"] = AutoScroll::kPage;
+  config_map_["DefaultViewerDivider"] = 2;
 
   config_map_["DiskCachePath"] = QStandardPaths::writableLocation(QStandardPaths::AppLocalDataLocation);
   config_map_["DiskCacheSize"] = 20.0;