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/llcorehttp/httprequest.h

@@ -103,9 +103,9 @@ class HttpRequest
 
 	/// Represents a default, catch-all policy class that guarantees
 	/// eventual service for any HTTP request.
-	static const policy_t DEFAULT_POLICY_ID = 0;
-	static const policy_t INVALID_POLICY_ID = 0xFFFFFFFFU;
-	static const policy_t GLOBAL_POLICY_ID = 0xFFFFFFFEU;
+	inline static constexpr policy_t DEFAULT_POLICY_ID = 0;
+	inline static constexpr policy_t INVALID_POLICY_ID = 0xFFFFFFFFU;
+	inline static constexpr policy_t GLOBAL_POLICY_ID = 0xFFFFFFFEU;
 
 	/// Create a new policy class into which requests can be made.
 	///