Fix PHP-Strict Warning

The warning was about passing only variables by reference.

Fix array keys to match actual form-input names

only one select is needed here

added new class to manage page's struct data

enforce lower case table names

properly handle data column creation

since we want to skip empty column names we need to create the data
columns one by one as well.

added JSON editor

This makes editing the configuration for fields nicer. Types have to
define a complete (possibly empty) default structure and the editor
allows to edit it then.

resorted code a bit

This separates the actual data holders from their editing tools.

Schema-Editor: made type a dropdown

some more PHP docs for better understanding

a very first basic version of the schema editor works

a first go at the overall structure. far from complete