HexEditor: Store annotations in a Model

A model is necessary for displaying a list of them in the UI. We might
as well make that their home.

Userland/Applications/HexEditor/AnnotationsModel.h

@@ -0,0 +1,70 @@
+/*
+ * Copyright (c) 2024, Sam Atkins <atkinssj@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Optional.h>
+#include <AK/Types.h>
+#include <AK/Vector.h>
+#include <LibGUI/Model.h>
+
+struct Annotation {
+    size_t start_offset { 0 };
+    size_t end_offset { 0 };
+    Gfx::Color background_color { Color::from_argb(0xfffce94f) };
+    String comments {};
+
+    bool operator==(Annotation const& other) const = default;
+};
+
+class AnnotationsModel final : public GUI::Model {
+public:
+    enum Column {
+        Start,
+        End,
+        Comments,
+    };
+
+    enum class CustomRole {
+        StartOffset = to_underlying(GUI::ModelRole::Custom) + 1,
+        EndOffset,
+        Comments,
+    };
+
+    virtual int row_count(GUI::ModelIndex const& index = GUI::ModelIndex()) const override
+    {
+        if (!index.is_valid())
+            return m_annotations.size();
+        return 0;
+    }
+
+    virtual int column_count(GUI::ModelIndex const& = GUI::ModelIndex()) const override
+    {
+        return 3;
+    }
+
+    virtual ErrorOr<String> column_name(int column) const override
+    {
+        switch (column) {
+        case Column::Start:
+            return "Start"_string;
+        case Column::End:
+            return "End"_string;
+        case Column::Comments:
+            return "Comments"_string;
+        }
+        VERIFY_NOT_REACHED();
+    }
+
+    virtual GUI::Variant data(GUI::ModelIndex const& index, GUI::ModelRole role = GUI::ModelRole::Display) const override;
+
+    void add_annotation(Annotation);
+    void delete_annotation(Annotation const&);
+    Optional<Annotation&> closest_annotation_at(size_t position);
+
+private:
+    Vector<Annotation> m_annotations;
+};