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/configuration/configure_web.ui

@@ -170,6 +170,22 @@
      </item>
     </layout>
    </item>
+   <item>
+     <widget class="QGroupBox" name="discord_group">
+      <property name="title">
+       <string>Discord Presence</string>
+      </property>
+      <layout class="QVBoxLayout" name="verticalLayout_21">
+       <item>
+        <widget class="QCheckBox" name="toggle_discordrpc">
+         <property name="text">
+          <string>Show Current Game in your Discord Status</string>
+         </property>
+        </widget>
+       </item>
+      </layout>
+     </widget>
+   </item>
    <item>
     <spacer name="verticalSpacer">
      <property name="orientation">