Linux - Replace legacy desktop integration with modern XDG-based integration

Replace manual placement of .desktop files with modern XDG-based install process.

Update .desktop file to leverage modern XDG functions (namely, the ability for an application to request to be ran on the system's most powerful GPU, useful for hybrid graphics setups).

Also provides provisions for launching the viewer on the iGPU and associating SLURLs via the desktop file, accessible by right-clicking the desktop shortcut.

indra/newview/linux_tools/install.sh

@@ -55,10 +55,13 @@ function homedir_install()
 
     prompt "Proceed with the installation? [Y/N]: "
     if [[ $? == 0 ]]; then
-	exit 0
+	    exit 0
+    fi
+    if [[ -z "$XDG_DATA_HOME" ]]
+        install_to_prefix "$XDG_DATA_HOME/alchemy-install" #$XDG_DATA_HOME is a synonym for $HOME/.local/share/ unless the user has specified otherwise (unlikely).
+    else
+        install_to_prefix "$HOME/.local/share/alchemy-install" #XDG_DATA_HOME not set, so use default path as defined by XDG spec.
     fi
-
-    install_to_prefix "$XDG_DATA_HOME/alchemy-install" #$XDG_DATA_HOME is a synonym for $HOME/.local/share/ unless the user has specified otherwise (unlikely).
 }
 
 function root_install()