LibGUI: Add GFile and base class GIODevice.

Working with the LibC API's is tedious, so let's add some comfy C++ API's.

LibGUI/GFile.h

@@ -0,0 +1,22 @@
+#pragma once
+
+#include <LibGUI/GIODevice.h>
+#include <AK/AKString.h>
+
+class GFile final : public GIODevice {
+public:
+    GFile() { }
+    explicit GFile(const String&);
+    virtual ~GFile() override;
+
+    String filename() const { return m_filename; }
+    void set_filename(const String& filename) { m_filename = filename; }
+
+    virtual bool open(GIODevice::OpenMode) override;
+    virtual bool close() override;
+
+    virtual const char* class_name() const override { return "GFile"; }
+
+private:
+    String m_filename;
+};