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.cpp

@@ -0,0 +1,89 @@
+#include <LibGUI/GIODevice.h>
+#include <unistd.h>
+
+GIODevice::GIODevice(GObject* parent)
+    : GObject(parent)
+{
+}
+
+GIODevice::~GIODevice()
+{
+}
+
+const char* GIODevice::error_string() const
+{
+    return strerror(m_error);
+}
+
+ByteBuffer GIODevice::read(int max_size)
+{
+    if (m_fd < 0)
+        return { };
+    if (!max_size)
+        return { };
+    auto buffer = ByteBuffer::create_uninitialized(max_size);
+    auto* buffer_ptr = (char*)buffer.pointer();
+    int remaining_buffer_space = buffer.size();
+    if (!m_buffered_data.is_empty()) {
+        int taken_from_buffered = min(remaining_buffer_space, m_buffered_data.size());
+        memcpy(buffer_ptr, m_buffered_data.data(), taken_from_buffered);
+        Vector<byte> new_buffered_data;
+        new_buffered_data.append(m_buffered_data.data() + taken_from_buffered, m_buffered_data.size() - taken_from_buffered);
+        m_buffered_data = move(new_buffered_data);
+        remaining_buffer_space -= taken_from_buffered;
+        buffer_ptr += taken_from_buffered;
+    }
+    if (!remaining_buffer_space)
+        return buffer;
+    int nread = ::read(m_fd, buffer_ptr, remaining_buffer_space);
+    if (nread < 0) {
+        set_error(errno);
+        return { };
+    }
+    buffer.trim(nread);
+    return buffer;
+}
+
+ByteBuffer GIODevice::read_line(int max_size)
+{
+    if (m_fd < 0)
+        return { };
+    if (!max_size)
+        return { };
+    auto line = ByteBuffer::create_uninitialized(max_size);
+    int line_index = 0;
+    while (line_index < line.size()) {
+        if (line_index >= m_buffered_data.size()) {
+            if (!populate_read_buffer())
+                return { };
+        }
+        byte ch = m_buffered_data[line_index];
+        line[line_index++] = ch;
+        if (ch == '\n') {
+            Vector<byte> new_buffered_data;
+            new_buffered_data.append(m_buffered_data.data() + line_index, m_buffered_data.size() - line_index);
+            m_buffered_data = move(new_buffered_data);
+            line.trim(line_index);
+            return line;
+        }
+    }
+    return { };
+}
+
+bool GIODevice::populate_read_buffer()
+{
+    if (m_fd < 0)
+        return false;
+    auto buffer = ByteBuffer::create_uninitialized(1024);
+    int nread = ::read(m_fd, buffer.pointer(), buffer.size());
+    if (nread < 0) {
+        set_error(errno);
+        return false;
+    }
+    if (nread == 0) {
+        set_eof(true);
+        return false;
+    }
+    m_buffered_data.append(buffer.pointer(), buffer.size());
+    return true;
+}