feat: add X-XR-Plugin flag to .desktop file

This change is part of a new initiative in Envision to revamp the plugin
system. The new system is such that Envision will no longer install plugins on
its own, instead it will detect system provided plugin applications via
the X-XR-Plugin flag, and be instructed on a specific command to use
with the optional X-XR-Plugin-Exec.

This proposed specification is detailed in the following issue:
<https://gitlab.com/gabmus/envision/-/issues/250>

Author: Gabriele Musco

org.stardustxr.Telescope.desktop

@@ -5,3 +5,4 @@ Exec=telescope
 Type=Application
 Icon=org.stardustxr.Telescope
 Categories=Utility;X-WiVRn-VR;
+X-XR-Plugin=true