Change name of visualizer target to simbody-visualizer.
Change GUI_NAME, etc. to  simbody-visualizer.

With only these changes, the examples fail to realize that the
VisualizerGUI executable has been renamed. This commit thus breaks the
invocation of simbody-visualizer by other executables.
Rename files and folders named VisualizerGUI.
Appropriately edit occurences of 'VisualizerGUI'.

The text 'VisualizerGUI' appears in no text files now. The only place it
exists is in the *.doc files. I don't want to edit those, because I
don't have Microsoft Word. I didn't do a simple find-replace from
VisualizerGUI to simbody-visualizer. In Doxygen documentation, I mostly
converted VisualizerGUI to visualizer. In Exception messages, I
converted to simbody-visualizer.