BrokerDiagram improvements, a.o. highlight the (real) current broker

When viewing the broker diagram with up to one or two master brokers (and optionally a slave broker for each) then it is unclear which broker is the current broker.
When there are 3 or more master brokers, the connections between the brokers provide a clue (as the one with multiple connections is from the current pair). but still it is unclear whether the master broker or the slave broker of the current pair is the current broker.

The PR for this change introduces highlighting of the current node and the display of spare slave brokers. Additionally, the previous implementation is fixed where needed.

Details:

• Remove comments about icons used for address/queue nodes as it does not even use an icon (I guess it might have done so in the past);
• Do not show ":61616" in the broker name, as that is the default;
• Do not show quotes around addresses and queues;
• The node-type "ThisBroker" is split into "ThisMasterBroker" and "ThisSlaveBroker". "ThisMasterBroker" has the same style as "MasterBroker", but just with a black border. "ThisSlaveBroker" has the same style as "SlaveBroker", but just with a black border. These styles are applied as appropriate;
• Only show the details when the current broker is selected. Previously the details were shown when the corresponding master broker was selected. That was correct when the current node was a master, but not when the current broker was a slave;
• When the current broker cannot be found in the reported topology, use the current broker-name instead of the text "broker". The brokers that are found in the topology are labelled with their hostname, so it is slightly different. The hostname does not seem to be available in the information that is retrieved for this diagram. But it's too much work to get the actual hostname as far as I can tell.
• When the connectors list shows brokers that are not part of the cluster, also show these brokers. the broker is drawn as a white circle with red border. Such brokers may be spare slave brokers (when there are more slave brokers than master brokers) or brokers that are not connected to yet.
• Do not show the checkbox controls for which the diagram will not change

The end-result still looks familiar in most cases, but note the slightly different border of the middle master broker:

In case spare slave brokers are used or when other brokers are just not connected: