Property class

Class to hold user-defined properties.

The Property type exposes the following members:

Property	Description
value	Gets or sets the value.
name	Gets the name of the property
value_type	Gets the type of the property value.

Method	Description
`get_extra(self, name)`	Gets extra data of the property associated by name.
`set_extra(self, name, value)`	Sets extra data of the property associated by name.
`get_bind_point(self, anim, create)`	Gets the property bind point on specified animation instance.
`get_keyframe_sequence(self, anim, create)`	Gets the keyframe sequence on specified animation instance.