Use default zoom level if no dir specific zoom level is set (#1104)

When directory specific settings are enabled, the default zoom level for the view-type will not be changed anymore.

This follows the principle used for directory specific sort column/order

If you want to change the default zoom level for a specific view-type, you either need to disable directory specific settings, or make the change via xfconf (xfce4-settings-editor)

(Later on we might offer these settings as well in the thunar preferences ... see !218)