Add Discord Rich Presence Support (#3883)

* Initial Discord RPC support

Build with Discord Presence ON

Fix RPC detection

Fix Time elapsed on pause; will now continue to count.

* Fix CI builds with compile flag

Addressed reviews

Fix silly mistakes

Fix 'Not in-game' display

class instead of namespace

Fix

Revamped

remove redundant code

Using Pimpl pattern

* Implement Null class

* Fix config updation

* Addressed All Reviews

* externals/discord-rpc : Updated to latest commit

src/citra_qt/ui_settings.h

@@ -58,6 +58,9 @@ struct Values {
     bool update_on_close;
     bool check_for_update_on_start;
 
+    // Discord RPC
+    bool enable_discord_presence;
+
     QString roms_path;
     QString symbols_path;
     QString game_dir_deprecated;