Feature request: Add CSS classes for different notification positions

Currently, there are no separate CSS classes for different notification position settings, so notification bubbles cannot be themed based on their location set in Preferences. I think this would be helpful for more complex themes.

For example, notifications could be referred to #XfceNotifyWindow.bottom-right, #XfceNotifyWindow.top-left, #XfceNotifyWindow.bottom-middle, etc.