fields.base: simplified render_container
fields: better handling of initial and default data
i) if initial data is set via constructor use that
ii) use default_value if defined in schema
iii) fallback: use default_value defined by field type

-clear() now sets data to default value
fields.base: more tests
added tests for fields.text and fields.textarea
fields.checkbox: fixed initial/default value handling and added tests
field.select: major overhaul of initial/default data handling, schema checking; added tests