Enable the plugin by default

This is needed for proper integration with MATE and Xfce.

tray_icon.plugin

@@ -7,3 +7,6 @@ Description=A tray icon to control Rhythmbox
 Authors=Mendhak <rhythmboxtray@mendhak.com>
 Copyright=Mendhak, GPL v3
 Website=http://mendhak.github.com/rhythmbox-tray-icon/
+
+[RB]
+InitiallyEnabled=true