HexEditor: Add annotations system

Allow the user to highlight sections of the edited document, giving them
arbitrary background colors. These annotations can be created from a
selection, or by manually specifying the start and end offsets.
Annotations can be edited or deleted by right-clicking them.

Any color can be used for the background. Dark colors automatically make
the text white for easier readability. When creating a new annotation,
we use whatever color the user last picked as this is slightly more
likely to be the one they want.

Icons contributed by Cubic Love.

Co-authored-by: Cubic Love <7754483+cubiclove@users.noreply.github.com>

Userland/Applications/HexEditor/EditAnnotationWidget.h

@@ -0,0 +1,23 @@
+/*
+ * Copyright (c) 2024, Sam Atkins <atkinssj@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibGUI/Widget.h>
+
+namespace HexEditor {
+
+class EditAnnotationWidget : public GUI::Widget {
+    C_OBJECT_ABSTRACT(GoToOffsetWidget)
+public:
+    static ErrorOr<NonnullRefPtr<EditAnnotationWidget>> try_create();
+    virtual ~EditAnnotationWidget() override = default;
+
+private:
+    EditAnnotationWidget() = default;
+};
+
+}