Fluid: restructuring and rejuvenation of the source code.

* Add classes for application and project
* Removed all globals from Fluid.h
* Extracting args and project history into their own classes
* Moving globals into Application class
* Initialize values inside headers for some classes.
* Undo functionality wrapped in a class inside Project.
* File reader and writer are now linked to a project.
* Avoid global project access
* Nodes (former Types) will be managed by a new Tree class.
* Removed static members (hidden globals) form Node/Fl_Type.
* Adding Tree iterator.
* Use nullptr instead of 0, NULL, or 0L
* Renamed Fl_..._Type to ..._Node, FL_OVERRIDE -> override
* Renaming ..._type to ...::prototype
* Splitting Widget Panel into multiple files.
* Moved callback code into widget panel file.
* Cleaning up Fluid_Image -> Image_asset
* Moving Fd_Snap_Action into new namespace fld::app::Snap_Action etc.
* Moved mergeback into proj folder.
* `enum ID` is now `enum class Type`.

1 files changed, 93 insertions, 0 deletions

diff --git a/fluid/proj/undo.h b/fluid/proj/undo.h
new file mode 100644
index 000000000..f87c747bd
--- /dev/null
+++ b/fluid/proj/undo.h
@@ -0,0 +1,93 @@
+//
+// Fluid Undo header for the Fast Light Tool Kit (FLTK).
+//
+// Copyright 1998-2025 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
+// file is missing or damaged, see the license at:
+//
+//     https://www.fltk.org/COPYING.php
+//
+// Please see the following page on how to report bugs and issues:
+//
+//     https://www.fltk.org/bugs.php
+//
+
+#ifndef undo_h
+#define undo_h
+
+#include <FL/filename.H>
+
+class Fl_Widget;
+
+namespace fld {
+
+class Project;
+
+namespace proj {
+
+class Undo
+{
+public:
+  
+  enum class OnceType {
+    ALWAYS = 0,
+    WINDOW_RESIZE
+  };
+
+  /// Link Undo class to this project.
+  Project &proj_;
+  /// Current undo level in buffer
+  int current_ = 0;
+  /// Last undo level in buffer
+  int last_ = 0;
+  // Maximum undo level used
+  int max_ = 0;
+  /// Last undo level that was saved
+  int save_ = -1;
+  // Undo checkpointing paused?
+  int paused_ = 0;
+  // Undo file path
+  char path_[FL_PATH_MAX] { };
+  // length w/o filename
+  unsigned int path_len_ = 0;
+  /// Suspend further undos of the same type
+  OnceType once_type_ = OnceType::ALWAYS;
+
+public:
+
+  // Constructor.
+  Undo(Project &p);
+  // Destructor.
+  ~Undo();
+
+  // Save current file to undo buffer
+  void checkpoint();
+  // Save undo buffer once until a different checkpoint type is called
+  int checkpoint(OnceType type);
+  // Clear undo buffer
+  void clear();
+  // Resume undo checkpoints
+  void resume();
+  // Suspend undo checkpoints
+  void suspend();
+  // Return the undo filename.
+  char *filename(int level);
+
+  // Redo menu callback
+  void redo();
+  // Undo menu callback
+  void undo();
+
+  // Redo menu callback
+  static void redo_cb(Fl_Widget *, void *);
+  // Undo menu callback
+  static void undo_cb(Fl_Widget *, void *);
+};
+
+} // namespace fld
+} // namespace proj
+
+
+#endif // !undo_h