Add Field.prototype.toXml() & .fromXml()

[AnmAtAnm]

Problem statement

Some fields need more than the value= attribute of the <field> XML when serializing and deserializing data. For example, FieldVariable uses id= and variabletype=.

Proposed Solution

• Add .fromXml(/* @type {!Element} */ fieldElement) and .toXml(!Element} */ fieldElement) to the Field base class.
• Modify Blockly.Xml.fieldToDom_(..) and .domToField_() to call new methods.

[RoboErikG]

After internal discussion we'd like to keep writing the value as the text in the element instead of adding a value property to the tag. Edited the checklist to avoid confusion.