LibGUI+PDFViewer: Move NumericInput into LibGUI

A text box that is restricted to numbers within a range, is generally
useful. Let's make it available for use!

Userland/Libraries/LibGUI/NumericInput.h

@@ -0,0 +1,35 @@
+/*
+ * Copyright (c) 2021, Matthew Olsson <mattco@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/NumericLimits.h>
+#include <LibGUI/TextBox.h>
+
+namespace GUI {
+
+class NumericInput final : public GUI::TextBox {
+    C_OBJECT(NumericInput)
+public:
+    virtual ~NumericInput() override = default;
+
+    Function<void(i32)> on_number_changed;
+
+    void set_min_number(i32 number);
+    void set_max_number(i32 number);
+    void set_current_number(i32 number, GUI::AllowCallback allow_callback = GUI::AllowCallback::Yes);
+
+private:
+    NumericInput();
+    void on_focus_lost();
+
+    bool m_needs_text_reset { false };
+    i32 m_current_number { 0 };
+    i32 m_min_number { NumericLimits<i32>::min() };
+    i32 m_max_number { NumericLimits<i32>::max() };
+};
+
+}