UI issues with the new monitor chooser dialog

Issue Description

Steps to Reproduce Issue

1. Use another locale than C/English
2. Plug in a second monitor for which a desktop icon layout is not yet configured

Expected Behavior

The dialog which is then shown has several issues, see the screenshot below:

1. The title is simply "xfdesktop"
2. The Pango format <b></b> is not applied in the question label
3. The label texts defined in the .glade file are not translated and are missing in the .po files
4. The monitor selection list includes duplicate monitors or at least lacks a clear indicator where they differ (reported now as #421)

Additional Information

CC: @kelnos as author of commit 93b9abd3.

Environment & Versions

• OS distribution and version: Arch Linux
• Overall Xfce version: 4.20
• xfdesktop version (git hash if not a release): 4.20.1
• Windowing system (X11/Wayland): X11
• Compositor and version (if Wayland): -
• GTK Theme: Arc-Darker
• Icon Theme: Faenza
• UI Scale Factor: 1.0