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

include/wx/cmdline.h

@@ -43,7 +43,8 @@ enum wxCmdLineEntryFlags
     wxCMD_LINE_PARAM_MULTIPLE   = 0x04, // the parameter may be repeated
     wxCMD_LINE_OPTION_HELP      = 0x08, // this option is a help request
     wxCMD_LINE_NEEDS_SEPARATOR  = 0x10, // must have sep before the value
-    wxCMD_LINE_SWITCH_NEGATABLE = 0x20  // this switch can be negated (e.g. /S-)
+    wxCMD_LINE_SWITCH_NEGATABLE = 0x20, // this switch can be negated (e.g. /S-)
+    wxCMD_LINE_HIDDEN           = 0x40  // this switch is not listed by Usage()
 };
 
 // an option value or parameter may be a string (the most common case), a