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/GIODevice.h

@@ -0,0 +1,53 @@
+#pragma once
+
+#include <LibGUI/GObject.h>
+#include <AK/ByteBuffer.h>
+
+class GIODevice : public GObject {
+public:
+    enum OpenMode {
+        NotOpen      = 0,
+        ReadOnly     = 1,
+        WriteOnly    = 2,
+        ReadWrite    = 3,
+        Append       = 4,
+        Truncate     = 8,
+        MustBeNew    = 16,
+    };
+
+    virtual ~GIODevice() override;
+
+    int fd() const { return m_fd; }
+    unsigned mode() const { return m_mode; }
+    bool eof() const { return m_eof; }
+
+    int error() const { return m_error; }
+    const char* error_string() const;
+
+    bool has_error() const { return m_error != 0; }
+
+    ByteBuffer read(int max_size);
+    ByteBuffer read_line(int max_size);
+
+    virtual bool open(GIODevice::OpenMode) = 0;
+    virtual bool close() = 0;
+
+    virtual const char* class_name() const override { return "GIODevice"; }
+
+protected:
+    explicit GIODevice(GObject* parent = nullptr);
+
+    void set_fd(int fd) { m_fd = fd; }
+    void set_mode(OpenMode mode) { m_mode = mode; }
+    void set_error(int error) { m_error = error; }
+    void set_eof(bool eof) { m_eof = eof; }
+
+private:
+    bool populate_read_buffer();
+
+    int m_fd { -1 };
+    int m_error { 0 };
+    bool m_eof { false };
+    OpenMode m_mode { NotOpen };
+    Vector<byte> m_buffered_data;
+};