Remove friend declaration from Fl_Window.H (see STR 3354)

Although this was not strictly necessary I decided to implement the
proposed patch, but in a slightly different way.

This adds a static public method for internal use only but it wouldn't
do any harm if a user called it.

FL/Fl_Window.H

@@ -1,7 +1,7 @@
 //
 // Window header file for the Fast Light Tool Kit (FLTK).
 //
-// Copyright 1998-2022 by Bill Spitzak and others.
+// Copyright 1998-2023 by Bill Spitzak and others.
 //
 // This library is free software. Distribution and use rights are outlined in
 // the file "COPYING" which should have been included with this file.  If this
@@ -53,12 +53,11 @@ class Fl_Double_Window;
   flags are ignored.
 */
 class FL_EXPORT Fl_Window : public Fl_Group {
-  friend int Fl::arg(int argc, char **argv, int &i);
   friend class Fl_X;
   friend class Fl_Window_Driver;
 private:
   static char *default_xclass_;
-  static char show_iconic_; // 1 means create next window in iconic form
+  static char show_next_window_iconic_; // 1 means create next window in iconic form
 
   int no_fullscreen_x;
   int no_fullscreen_y;
@@ -602,6 +601,28 @@ public:
   void screen_num(int screen_num);
   static bool is_a_rescale();
   fl_uintptr_t os_id();
+
+  /**
+    Sets a static flag whether the next window should be opened iconified.
+
+    \note This is an <b>internal function</b>, you should not use this in user code.
+
+    Please use Fl_Window::iconize() instead.
+  */
+  static void show_next_window_iconic(char stat) {
+    show_next_window_iconic_ = stat ? 1 : 0;
+  }
+
+  /**
+    Returns the static flag whether the next window should be opened iconified.
+
+    \note This is an <b>internal function</b>, you should not use this in user code.
+
+    Please use Fl_Window::iconize() to iconify a window.
+  */
+  static char show_next_window_iconic() {
+    return show_next_window_iconic_;
+  }
 };
 
 #endif

src/Fl_Window.cxx

@@ -35,7 +35,7 @@
 
 char *Fl_Window::default_xclass_ = 0L;
 
-char Fl_Window::show_iconic_ = 0;
+char Fl_Window::show_next_window_iconic_ = 0;
 
 Fl_Window *Fl_Window::current_;
 

src/Fl_Window_Driver.H

@@ -114,8 +114,8 @@ public:
   void x(int X);
   void y(int Y);
   void current(Fl_Window *c);
-  char show_iconic() { return Fl_Window::show_iconic_; }
-  void show_iconic(char c) { Fl_Window::show_iconic_ = c; }
+  char show_iconic() { return Fl_Window::show_next_window_iconic(); }
+  void show_iconic(char c) { Fl_Window::show_next_window_iconic(c); }
   void flx(Fl_X *x) { pWindow->flx_ = x; }
   Fl_Cursor cursor_default() { return pWindow->cursor_default; }
   void destroy_double_buffer();

src/Fl_Window_iconize.cxx

@@ -1,7 +1,7 @@
 //
 // Window minification code for the Fast Light Tool Kit (FLTK).
 //
-// Copyright 1998-2010 by Bill Spitzak and others.
+// Copyright 1998-2023 by Bill Spitzak and others.
 //
 // This library is free software. Distribution and use rights are outlined in
 // the file "COPYING" which should have been included with this file.  If this
@@ -19,7 +19,7 @@
 
 void Fl_Window::iconize() {
   if (!shown()) {
-    show_iconic_ = 1;
+    show_next_window_iconic(1);
     show();
   } else {
     pWindowDriver->iconize();

src/Fl_arg.cxx

@@ -126,7 +126,7 @@ int Fl::arg(int argc, char **argv, int &i) {
   s++; // point after the dash
 
   if (fl_match(s, "iconic")) {
-    Fl_Window::show_iconic_ = 1;
+    Fl_Window::show_next_window_iconic(1);
     i++;
     return 1;
   } else if (fl_match(s, "kbd")) {

src/Fl_x.cxx

@@ -2619,10 +2619,10 @@ void Fl_X::make_xid(Fl_Window* win, XVisualInfo *visual, Colormap colormap)
     XWMHints *hints = XAllocWMHints();
     hints->input = True;
     hints->flags = InputHint;
-    if (Fl_Window::show_iconic_) {
+    if (Fl_Window::show_next_window_iconic()) {
       hints->flags |= StateHint;
       hints->initial_state = IconicState;
-      Fl_Window::show_iconic_ = 0;
+      Fl_Window::show_next_window_iconic(0);
       showit = 0;
     }
     if (Fl_X11_Window_Driver::driver(win)->icon_ &&