Move the Fl_String class to the DriverDeveloper Doxygen group

src/Fl_String.H

@@ -17,6 +17,12 @@
 #ifndef _FL_Fl_String_H_
 #define _FL_Fl_String_H_
 
+/**
+ \cond DriverDev
+ \addtogroup DriverDeveloper
+ \{
+ */
+
 /** \file src/Fl_String.H
   Basic Fl_String class for FLTK.
 */
@@ -130,4 +136,9 @@ FL_EXPORT Fl_String operator+(const Fl_String& lhs, const Fl_String& rhs);
 FL_EXPORT Fl_String operator+(const Fl_String& lhs, const char* rhs);
 FL_EXPORT bool operator==(const Fl_String & lhs, const Fl_String & rhs);
 
+/**
+\}
+\endcond
+*/
+
 #endif // _FL_Fl_String_H_

src/Fl_String.cxx

@@ -14,6 +14,12 @@
 //     https://www.fltk.org/bugs.php
 //
 
+/**
+ \cond DriverDev
+ \addtogroup DriverDeveloper
+ \{
+ */
+
 #include "Fl_String.H"
 
 #include <stdio.h>
@@ -637,3 +643,8 @@ bool operator==(const Fl_String &lhs, const Fl_String &rhs) {
   }
   return false;
 }
+
+/**
+\}
+\endcond
+*/