KCM Fonts .fonts.conf was not updated and enforce consistency through UI

Summary:

• If some elements in .fonts.conf were deleted (default), they weren't updated anymore, and cause inconsistency between the value displayed in the KCM and what is actually applied.

• UI consistency : when the user uncheck enable AA, then the style is set to None instead of default value. Also, when the user selects None, the checkbox enable AA is unchecked.

CCBUG: 416140

Reviewers: bport, meven, ngraham, davidedmundson, #plasma

Reviewed By: ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D29402