Fix reset to defaults in the fonts KCM

Summary:
Turns out one of the properties was utterly broken emitting the wrong
signal, and the defaults() method was missing a piece of state to reset.

Since it was making my eyes bleed I also removed the "const int &"
parameters in the process. :-)

Reviewers: #plasma, bport, davidedmundson, mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25094