[IceProxyFinder widget] Add a signal which is emitted when a valid proxy is selected

Scope: IceProxyFinder widget in libraries/ArmarXGuiBase/widgets.

Change summary: Added a signal which is emitted when a valid proxy is selected by the user, be it via the combobox dropdown functionality or its line edit functionality (evaluates on the fly)

Rationale: The IceProxyFinder widget is usually used within configuration pages, as for example in VisionX' ImageMonitor widget. Given a selected Ice proxy, it is sometimes desirable to make further configurations depending on the concrete Ice proxy selected. This new signal allows for a connection so the rest of the UI can be updated when the user selects a valid proxy. This is exactly my use case, as I am working on an improvement for VisionX (its ImageMonitor component).