Refactor callback to signal when object selection changes
Currently, when StelObjectMgr registers that the selected object(s) change, it calls a method defined in StelModule, and implemented in StelModules subclasses. The refactoring is to change this registered callabcks to the more standard signal/slot metaphor of Qt.
Blueprint information
Whiteboard
The reason for this refactoring is the help move to a more fully Qt design, utilizing the signal/slot metaphor for notifications. There are several places in the code where this is needed; I chose to start here as LogBook will need to know when the object selection changes, and even though it is a StelModule, the actual class that needs this information is not. So the option was to put it code that would have been a kludge to pass this on to where it was needed, or, directly connect a slot to the signal that the object selection has changed.
