Add support to garcon's PrefersNonDefaultGPU

If a desktop file contains PrefersNonDefaultGPU=true thunar should launch it with the following env vars:

• __NV_PRIME_RENDER_OFFLOAD=1
• __GLX_VENDOR_LIBRARY_NAME=nvidia
• __VK_LAYER_NV_optimus=NVIDIA_only

For more details see:

• xfdesktop#81 (closed)
• garcon#21 (closed)
• garcon!16 (merged)
• xfce4-appfinder!17 (merged)