add support for xrandr hotplug_mode_update property / SPICE resize support

Submitted by Martin

Assigned to Xfce Bug Triage

Description

Dear developers,

I use a SPICE viewer [1] to connect to a machine running XFCE 4.12 (Debian Buster). Unfortunately spice-vdagent altered its behaviour from changing screen resolution itself to just informing the Window manager / Desktop Environment of the resolution change ("hotplug_mode_update"). So automatic resizing of all components in the viewer window fails. Other DE (KDE and GNOME) change screen resolution without problems, as they somehow fixed that problem.

Several bugs were filed in the last years [2], but the problem persists.

Could you please fix this, as I like the snappiness of xfce on poorly equipped/older machines and I do not want to switch to another DE.

Thanks & Regards Martin

[1] https://virt-manager.org/download/sources/virt-viewer/virt-viewer-x64-8.0.msi

[2] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=935896 (for xfce4 package) https://bugzilla.redhat.com/show_bug.cgi?id=1290586#c7 (for spice-vdagent package) https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=858549#15 (for spice-vdagent package)