Allow hiding command line arguments from Usage()

Add wxCMD_LINE_HIDDEN wxCmdLineParser flag allowing to hide options and/or
parameters.

A hidden/unlisted argument is processed as usual, but not shown in the output
given by Usage(). A use case for such could be diagnostics switches that
should exist but are not useful to the end user.

Closes https://github.com/wxWidgets/wxWidgets/pull/390

samples/console/console.cpp

@@ -41,6 +41,8 @@ static const wxCmdLineEntryDesc cmdLineDesc[] =
     { wxCMD_LINE_SWITCH, "h", "help", "show this help message",
         wxCMD_LINE_VAL_NONE, wxCMD_LINE_OPTION_HELP },
     { wxCMD_LINE_SWITCH, "d", "dummy", "a dummy switch" },
+    { wxCMD_LINE_SWITCH, "s", "secret", "a secret switch",
+        wxCMD_LINE_VAL_NONE, wxCMD_LINE_HIDDEN },
     // ... your other command line options here...
 
     { wxCMD_LINE_NONE }
@@ -96,6 +98,11 @@ int main(int argc, char **argv)
                         wxPrintf("Bad luck!\n");
                 }
             }
+            if (parser.Found("s"))
+            {
+                wxPrintf("Secret switch was given...\n");
+            }
+
             break;
 
         default: