Make schema dictionaries case insensitive

[janssenhenning]

some parts of the schema dictionary could be made case insensitive to make it easier to find attributes or tags

For this the following needs to done:

• Attribute paths have to include the attribute name with @ explicitely
• make sure that nowhere information is overwritten

This is only relevant for the entries attrib_types, unique_attribs, unique_path_attribs, other_attribs, tag_paths

[janssenhenning]

Note to myself: To make this easier it might be useful to convert the parts supposed to be case insensitive into a special subclass of dict to make the lookup case insensitive naturally. otherwise all kinds of direct lookup would need a .lower() or a function to mask it