forbid sub-rules

Summary:
sub-rules are a hassle

they are only allowed for ints and floats

they can easily replaced with an extra context

I will patch all .xml files and forbid that, will make the implementation
much nicer, as that is the only place we allow any "endless" nesting
in the implementation

Test Plan:
make && make test

> this is only the first part, need to finish all remaining .xml files

Reviewers: vkrause, dhaumann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D15219