Add an is on channel method to channel.

Issue 3811

Change-Id: I1660850b96b56fbb3d3a47441a504295d38ed1ec
Reviewed-on: http://gerrit.dmdirc.com/922
Reviewed-by: Chris Smith <chris@dmdirc.com>
Automatic-Compile: DMDirc Local Commits <dmdirc@googlemail.com>

src/com/dmdirc/Channel.java

@@ -262,6 +262,15 @@ public class Channel extends MessageTarget implements ConfigChangeListener {
         return tabCompleter;
     }
 
+    /**
+     * Are we on the channel?
+     *
+     * @return true iff we're on the channel
+     */
+    public boolean isOnChannel() {
+        return onChannel;
+    }
+
     /**
      * Called when we join this channel. Just needs to output a message.
      */