Add Subsonic scrobble support (#545)

* add SubsonicScrobbler, add Scrobble method in SubsonicService

* new class SubsonicScrobbleRequest, use queue again, clean up

* add checkbox to enable server-side scrobbling to Subsonic settings page

* Check serversidescrobbling in SubsonicScrobbler::ReloadSettings instead of SubsonicService

TODO: SubsonicScrobbler::ReloadSettings needs to be called when
SubsonicSettings change.

src/settings/subsonicsettingspage.ui

@@ -120,6 +120,13 @@
         </property>
        </widget>
       </item>
+      <item>
+       <widget class="QCheckBox" name="checkbox_server_scrobbling">
+        <property name="text">
+         <string>Server-side scrobbling</string>
+        </property>
+       </widget>
+      </item>
      </layout>
     </widget>
    </item>