Schema-Editor: made type a dropdown

added table to hold schema assignments

Schemas should be assignable to namespaces or pages. That's why the
column to hold that info is called assign and not pid.

added some more beef to the admin view

Now all existing schemas are listed in the TOC and a form allows adding
new ones.

Add first test for simple function

SPR-290

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

initial wizard generated structure