Make FieldLoader/FieldSaver able to work with private fields

* Unify the FieldLoader-related attributes (Ignore, LoadUsing,
FieldFromYamlKey) into a new 'Serialize' attribute. Keep the
previous attributes as shortcuts/aliases to the new attribute.

* Add 'YamlName' field to the new 'Serialize' attribute to allow
yaml names different from the field names. Example:
    [Serialize(YamlName = "Width")]
    private int widthExpression = "0";

* The FieldLoader will treat private fields like public now and
will try to load their values. The FieldSaver gets an optional
parameter specifying whether to save private fields by default.
The 'Serialize' attribute can be used to fine-tune individual
fields.

* Updated the traits documentation exporter to write the yaml
name if it has been overriden.
Trivial typo in exception message
Use available TypeConverters to serialize and deserialize types
Split Widget.cs for each class
Add Padding struct
Add initial box layout implementation