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, 106 insertions, 0 deletions

diff --git a/fluid/nodes/Tree.h b/fluid/nodes/Tree.h
new file mode 100644
index 000000000..f494af651
--- /dev/null
+++ b/fluid/nodes/Tree.h
@@ -0,0 +1,106 @@
+//
+// Node Tree header file 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 FLUID_NODES_TREE_H
+#define FLUID_NODES_TREE_H
+
+#include "nodes/Widget_Node.h"
+
+class Node;
+
+namespace fld {
+
+class Project;
+
+namespace node {
+
+class Tree {
+
+  // A class that can iterate over the entire scene graph.
+  class Iterator {
+    Node *type_ = nullptr;
+    bool only_selected_ = false;
+  public:
+    explicit Iterator(Node *t, bool only_selected);
+    Node* operator*() { return type_; }
+    Iterator& operator++();
+    bool operator!=(const Iterator& other) const { return type_ != other.type_; }
+  };
+
+  // A container for a node iterator
+  class Container {
+    Tree &tree_;
+    bool only_selected_ = false;
+  public:
+    Container(Tree &tree, bool only_selected) : tree_(tree), only_selected_(only_selected) { }
+    Iterator begin() { return Iterator(tree_.first, only_selected_); }
+    Iterator end() { return Iterator(nullptr, only_selected_); }
+  };
+
+  // A class that iterate over the scene graph, but returns only nodes of type widget.
+  class WIterator {
+    Node *type_ = nullptr;
+    bool only_selected_ = false;
+  public:
+    explicit WIterator(Node *t, bool only_selected);
+    Widget_Node* operator*() { return static_cast<Widget_Node*>(type_); }
+    WIterator& operator++();
+    bool operator!=(const WIterator& other) const { return type_ != other.type_; }
+  };
+
+  // A container for a widget node iterator
+  class WContainer {
+    Tree &tree_;
+    bool only_selected_ = false;
+  public:
+    WContainer(Tree &tree, bool only_selected) : tree_(tree), only_selected_(only_selected) { }
+    WIterator begin() { return WIterator(tree_.first, only_selected_); }
+    WIterator end() { return WIterator(nullptr, only_selected_); }
+  };
+
+  /// Link Tree class to the project.
+  Project &proj_;
+
+public:
+
+  Node *first = nullptr;
+  Node *last = nullptr;
+  Node *current = nullptr;  // most recently picked object
+  Node *current_dnd = nullptr;
+  /// If this is greater zero, widgets will be allowed to lay out their children.
+  int allow_layout = 0;
+
+public:
+
+  Tree(Project &proj);
+
+  bool empty() { return first == nullptr; }
+
+  // Iterators: `for (auto &n: tree.all_nodes()) { n.print(); }
+  Container all_nodes() { return Container(*this, false); }
+  WContainer all_widgets() { return WContainer(*this, false); }
+  Container all_selected_nodes() { return Container(*this, true); }
+  WContainer all_selected_widgets() { return WContainer(*this, true); }
+
+  Node *find_by_uid(unsigned short uid);
+  Node *find_in_text(int text_type, int crsr);
+};
+
+} // namespace node
+} // namespace fld
+
+
+#endif // FLUID_NODES_TREE_H