Parameterize Property and Field with the value type.

Parameterize Property and Field with the value type.

The interfaces Property and Field and most of their implementations are
now parameterized with the type of their value. The method getValue()
returns the Property type but setValue() takes Object as its parameter.
No implicit conversions between value type and strings are performed in
most locations (fields).

Among others, AbstractTextField, DateField and RichTextArea not have
specific value types and do not accept arbitrary values.

Most locations requiring migration will be visible as compilation
errors, with the exception of some cases where a non-parameterized
Property or Field (or one parametrized with Object) is used.

Not yet done:
- Label
- converters
- setValue() parameterization (causes much more migration effort)