Fix zenity invocation for KDE

[jreichel-nvidia]

The KDE file open dialog hangs because of the trailing space.

[jreichel-nvidia]

Can someone take care of NanoGUI which has the same problem (affects MaterialXView)?

[jstone-lucasfilm]

Thanks for the fix, @jreichel-nvidia!

[jreichel-nvidia]

For reference: In newer versions of NanoGUI the problem has been fixed here:

commit 1cbff2e8d79507e77ab000ee5282c35694689ade
Author: Wenzel Jakob wenzel.jakob@epfl.ch
Date: Fri Aug 29 18:55:10 2025 +0200

Switch file open/save dialog provider to ``nativefiledialog-extended``

This commit replaces the hand-written file dialog code in common.cpp
with the library https://github.com/btzy/nativefiledialog-extended.

The main consequence of this change is that the Linux implementation of
these operations switches from zenity to xdg-desktop-portal, causing the
file dialogs to also work in containers (e.g., flatpak) and non-GTK
based distributions like KDE. The library also has explicit support for
Wayland.

[jstone-lucasfilm]

Ah, that's good to know, @jreichel-nvidia. I'm glad Wenzel is still making improvements to NanoGUI, and we should update our submodule reference to the latest commit when time permits.