From Eric Sokolowski and Robert Osfield, moved command line option usage setup

from osgviewer example into osg::ArgumentParser and osgViewer::Viewer to make
them more universally available.

include/osg/ApplicationUsage

@@ -43,9 +43,11 @@ class OSG_EXPORT ApplicationUsage : public osg::Referenced
 
         enum Type
         {
+            NO_HELP = 0x0,
             COMMAND_LINE_OPTION = 0x1,
             ENVIRONMENTAL_VARIABLE = 0x2,
-            KEYBOARD_MOUSE_BINDING = 0x4
+            KEYBOARD_MOUSE_BINDING = 0x4,
+            HELP_ALL = KEYBOARD_MOUSE_BINDING|ENVIRONMENTAL_VARIABLE|COMMAND_LINE_OPTION
         };
         
         void addUsageExplanation(Type type,const std::string& option,const std::string& explanation);

include/osg/ArgumentParser

@@ -190,6 +190,14 @@ class OSG_EXPORT ArgumentParser
         /** Write error messages to the given ostream, if at or above the given severity. */
         void writeErrorMessages(std::ostream& output,ErrorSeverity sevrity=BENIGN);
 
+        
+        /** This convinience method handles help requests on the command line.
+          * Return the type(s) of help requested. The return value of this
+          * function is suitable for passing into getApplicationUsage()->write().
+          * If ApplicationUsage::NO_HELP is returned then no help commandline option 
+          * was found on the command line. */
+        ApplicationUsage::Type readHelpType();
+
   
   protected: