Update to use StatusService in its new location

The StatusService moved to org.scijava.app (from org.scijava.event) and
gained a new method, getStatusMessage(StatusEvent), which returns a
reasonable default status message in the case of an empty message ("").

This means that the ImageJ status bar is now never blank, but instead
shows some reasonable default information about the ImageJ application
version, etc.

See ticket #1753 for further details.
Clear tool description on mouse exit