Fix DnD to Wayland FLTK app on the KDE/Plasma desktop

src/drivers/Wayland/fl_wayland_clipboard_dnd.cxx

@@ -1,7 +1,7 @@
 //
 // Wayland-specific code for clipboard and drag-n-drop support.
 //
-// Copyright 1998-2023 by Bill Spitzak and others.
+// Copyright 1998-2024 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
@@ -297,6 +297,9 @@ static void data_offer_handle_offer(void *data, struct wl_data_offer *offer,
   } else if (strcmp(mime_type, wld_plain_text_clipboard) == 0 && !fl_selection_type[1]) {
     fl_selection_type[1] = Fl::clipboard_plain_text;
     fl_selection_offer_type = wld_plain_text_clipboard;
+  } else if (strcmp(mime_type, "text/plain") == 0 && !fl_selection_type[1]) {
+    fl_selection_type[1] = Fl::clipboard_plain_text;
+    fl_selection_offer_type = "text/plain";
   }
 }