kcm_fonts: Fix tracking of configuration changes

Summary:
Sometimes, kcm_fonts wrote garbage into fonts.conf. This was a result of not
initializing variables properly. In addition, toggling the exclude range
checkbox did not enable the Apply button. That's fixed as well now.

Test Plan:
Did almost every possible antialiasing settings configuration,
fonts.conf looked as expected.

Reviewers: Plasma, bshah

Reviewed By: Plasma, bshah

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D21640