IRCClient: Implement the "part from channel" action.

Also make sure the action is disabled while we're not in a window that
corresponds to an open channel. :^)

Fixes #277.

Applications/IRCClient/IRCClient.cpp

@@ -647,3 +647,9 @@ void IRCClient::handle_part_action(const String& channel)
 {
     part_channel(channel);
 }
+
+void IRCClient::did_part_from_channel(IRCChannel& channel)
+{
+    if (on_part_from_channel)
+        on_part_from_channel(channel);
+}