kcm/fonts: use correct signal handlers for UI controls
Reason for the change
Using the on[Thing]Changed() versions can cause loops and unexpected state changes, based on how the backend may be implemented. Instead use the versions that only fire on explicit user interaction.
Test plan
Use all touched UI elements in the KCM and verify that they all work.
Screenshots or screen recordings
N/A
Bugs fixed
May fix https://bugs.kde.org/show_bug.cgi?id=486699 based on code inspection, but I can't reproduce it