GIODevice: Add a read_all() that returns a ByteBuffer with all we can read.

Use this to implement file opening in TextEditor.

AK/String.cpp

@@ -92,6 +92,15 @@ ByteBuffer String::to_byte_buffer() const
     return ByteBuffer::copy(reinterpret_cast<const byte*>(characters()), length());
 }
 
+String String::from_byte_buffer(const ByteBuffer& buffer)
+{
+    if (buffer.is_null())
+        return nullptr;
+    if (buffer.is_empty())
+        return empty();
+    return String((const char*)buffer.pointer(), buffer.size());
+}
+
 unsigned String::to_uint(bool& ok) const
 {
     unsigned value = 0;