Added starter implementation.

.gitignore

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+.dub
+docs.json
+__dummy.html
+docs/
+/todo-d
+todo-d.so
+todo-d.dylib
+todo-d.dll
+todo-d.a
+todo-d.lib
+todo-d-test-*
+*.exe
+*.o
+*.obj
+*.lst

README.md

@@ -1,2 +1,4 @@
 # todo-d
 A simple to-do list, implemented as a desktop application in D.
+
+This project is built using GTK-D, and as such, please ensure that you've installed the necessary prerequisites to be able to build and run this application.

dub.json

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+{
+	"authors": [
+		"Andrew Lalis"
+	],
+	"copyright": "Copyright © 2022, Andrew Lalis",
+	"dependencies": {
+		"gtk-d": "~>3.10.0"
+	},
+	"description": "A simple to-do list, implemented as a desktop application in D.",
+	"license": "MIT",
+	"name": "todo-d"
+}

dub.selections.json

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+{
+	"fileVersion": 1,
+	"versions": {
+		"gtk-d": "3.10.0"
+	}
+}

source/app.d

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+import std.stdio;
+
+import model;
+import view : setupUI;
+
+import gtk.MainWindow;
+import gtk.Main;
+
+void main(string[] args) {
+	Main.init(args);
+	MainWindow window = new MainWindow("To-Do");
+	window.setDefaultSize(300, 500);
+	setupUI(window);
+	window.showAll();
+	Main.run();
+}

source/model.d

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+module model;
+
+struct ToDoItem {
+    string text;
+    int priority;
+}

source/view.d

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+module view;
+
+import model;
+
+import gtk.Box;
+import gtk.ListBox;
+import gtk.ListBoxRow;
+import gtk.Label;
+import gtk.CheckButton;
+import gtk.Window;
+import gtk.ScrolledWindow;
+import gtk.Entry;
+import gtk.Button;
+
+// Yes, I use global items for my application.
+
+ListBox todoList;
+Entry todoEntry;
+
+ToDoItem[] items = [];
+
+void setupUI(Window w) {
+    todoList = new ListBox();
+    todoEntry = new Entry();
+
+	auto vbox = new Box(GtkOrientation.VERTICAL, 5);
+
+    todoList.setSelectionMode(GtkSelectionMode.NONE);
+
+	vbox.packStart(new ScrolledWindow(todoList), true, true, 5);
+
+    auto addBox = new Box(GtkOrientation.HORIZONTAL, 5);
+    addBox.packStart(todoEntry, true, true, 0);
+    Button addButton = new Button("Add");
+    addButton.addOnClicked(delegate(Button b) {addItem();});
+    addBox.packEnd(addButton, false, false, 0);
+    vbox.packEnd(addBox, false, false, 0);
+
+	w.add(vbox);
+}
+
+void addItem() {
+    import std.algorithm;
+    import std.string;
+    string text = todoEntry.getText().strip;
+    if (text.length == 0) return;
+    int maxPrio = -1_000_000;
+    foreach (item; items) {
+        maxPrio = max(maxPrio, item.priority);
+    }
+    ToDoItem newItem = ToDoItem(text, maxPrio + 1);
+    items ~= newItem;
+    todoEntry.setText("");
+    normalizePriorities();
+    refreshList();
+}
+
+void normalizePriorities() {
+    int prio = 1;
+    foreach (item; items) {
+        item.priority = prio++;
+    }
+}
+
+void removeItem(int prio) {
+    import std.algorithm;
+    import std.array;
+    ToDoItem[] newItems = items.filter!(item => item.priority != prio).array;
+    items = newItems;
+    normalizePriorities();
+    refreshList();
+}
+
+void refreshList() {
+    todoList.removeAll();
+    foreach (item; items) {
+        auto widget = new ToDoItemWidget(item);
+        auto row = new ListBoxRow();
+        row.add(widget);
+		todoList.add(row);
+	}
+    todoList.showAll();
+}
+
+class ToDoItemWidget : Box {
+    private ToDoItem item;
+
+    this(ToDoItem item) {
+        super(GtkOrientation.HORIZONTAL, 5);
+        this.item = item;
+        CheckButton button = new CheckButton();
+        Label label = new Label(item.text);
+        label.setLineWrap(true);
+        label.setHalign(GtkAlign.START);
+        label.setValign(GtkAlign.CENTER);
+        this.packStart(button, false, false, 0);
+        
+        Button removeButton = new Button("Remove");
+        removeButton.addOnClicked(delegate(Button b) {
+            removeItem(item.priority);
+        });
+        this.packEnd(removeButton, false, false, 0);
+        this.packEnd(label, true, true, 0);
+    }
+}