warn when risk of broadcasting on wrong bus

[jflamy]

If announcer is on red platform, and selects by mistake the blue platform that is currently running, there will be two announcers effectively announcing the blue.
A warning should be given when selecting a different platform than the currently selected one.

This scenario is legitimate when a scheduling constraint or platform repairs causes a platform move, so it should not be disabled. However, the sorting order for the blue announcer should probably be blue platforms not done, then blue done, then the remaining.