[ADD] initial theme guide + scaffolding

incomplete, incorrect, committed for testing
[FIX] fields: avoid prefetching of one2many fields

When reading a one2many field, the inverse mapping of the lines (matching m2o -> lines of corresponding record) was instantiating each line and then triggering the prefect of fields.
To improve the performances, the inverse mapping is done in sql to avoid triggering the prefetching.
[IMP] fields: update inverse fields lazily for performance

When a relational field is assigned in an onchange, its inverse field is
updated in cache.  Reading the current value of the inverse field may be
costly, for instance in the case of a one2many field with thousands of records
as a value.  Instead, put in cache a SpecialValue that reads and updates the
field; it will be triggered only when it is accessed.
[IMP] models: do not prefetch too many records at once
[IMP] models: prefetch fields with groups (those to which user has access)