Applications: Add Presenter

This version can already:
- load all of the defined file format except for the image type and the
  frame-specific stuff
- navigate frames and slides (though frames are mostly stubbed out)
- display text with various common settings
- displays text with various fitting and scaling methods
- scale and position objects correctly no matter the window size

Userland/Applications/Presenter/PresenterWidget.h

@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) 2022, kleines Filmröllchen <filmroellchen@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include "LibGUI/Action.h"
+#include "Presentation.h"
+#include <LibGUI/Event.h>
+#include <LibGUI/UIDimensions.h>
+#include <LibGUI/Widget.h>
+
+// Title, Author
+constexpr StringView const title_template = "{} ({}) — Presenter"sv;
+
+class PresenterWidget : public GUI::Widget {
+    C_OBJECT(PresenterWidget);
+
+public:
+    PresenterWidget();
+    ErrorOr<void> initialize_menubar();
+
+    virtual ~PresenterWidget() override = default;
+
+    // Errors that happen here are directly displayed to the user.
+    void set_file(StringView file_name);
+
+protected:
+    virtual void paint_event(GUI::PaintEvent&) override;
+    virtual void keydown_event(GUI::KeyEvent&) override;
+
+private:
+    OwnPtr<Presentation> m_current_presentation;
+    RefPtr<GUI::Action> m_next_slide_action;
+    RefPtr<GUI::Action> m_previous_slide_action;
+};