xfce4-notifyd plugin: selecting notification settings a second time should not open a second window

Issue Description

Steps to Reproduce Issue

1. Open the xfce4notifyd plugin's menu and select "notification settings"
2. The "notifications" settings window comes up.
3. Select "notification settings" from the xfce4notifyd plugin's menu once more
4. A second "notifications" settings window comes up.

Expected Behavior

Selecting "notification settings" while that window is still open should bring that window to foreground, but not open a second instance (see for example how the Pulse Audio plugin does it with the Mixer window)

Versions

• OS distribution and version: Manjaro latest stable
• Overall Xfce version: 4.20
• xfce4-notifyd version (git hash if not a release): 0.9.7
• Windowing system (X11/Wayland): should not matter
• Compositor and version (if Wayland): should not matter