ORM: Implement 'nullable' and 'required' column flags

[ghengeveld]

To allow simple field validation with a sensible error message, a 'required' flag should be added as alternative to 'minlength' => 1. Default value should be false.

In createTable() there is already a 'null' flag implemented, but it's only used to set the MySQL column flag NULL or NOT NULL. It makes more sense to rename this to 'nullable', because it can also be not null. Furthermore this flag should allow 'required' or 'minlength' > 0 columns to accept null as a valid value.