960 show "nullable" checkbox for highlighted attributes in schema diagram view

[Logende]

What was done:

• show "nullable" checkbox for highlighted attributes in schema diagram view
• add new setting for schema diagram on whether to show nullable checkbox
• adjust meta schema builder to allow [ , ] even when the simplification of allowing only one type is enabled
• add tests for new changes, as well as for existing meta schema builder functionality and schema manipulation utils

Why:

• user comfort: ability to make attributes nullable just be schema diagram
• better code quality