reused tiddler model's fields listing for text serialization

@cdent will probably hate this big time, but duplicating model
attributes all over the place seems undesirable

note that I did not adjust the JSON serialization accordingly because
things seem subtly different there (whitelisting a select set of
attributes when deserializing)
Merge branch 'text-serialization-refactor' into full-text-serialization

Conflicts:
	tiddlyweb/serializations/text.py
restored empty fields in text serialization

... while omitting them in the text store's representations - this
required introducing non-standard arguments, thus the text store now
accessed `tiddler_as` directly rather than through the `Serializer`
interface
Merge branch 'text-serialization-refactor' into full-text-serialization

note that this makes some tests fail - which is expected though; code
adjustments to follow

Conflicts:
	test/test_serialize_tiddler.py
	tiddlyweb/stores/text.py
added omit_fields option to text serialization

this allows the text store to omit the creator field, which is
calculated rather than persisted in that context

cf. 99a3d574f0b478546bde56994c39d4a9324caa12