::match and ::doMatch => const

Summary:
make ::match and ::doMatch const
ATM the keyword list is still mutable (internal it uses mutable, too)

for isDynamic, we just construct a new regex if needed
that is nicer than working on the stored one

Test Plan: make && make test

Reviewers: vkrause, dhaumann

Reviewed By: vkrause

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D15211