Fix BUG#105797: inappropriate fontconfig settings are saved when kcontrol/fonts is shown and no way to revert them

Fix kcontrol/fonts saves fontconfig settings when it is opened. Add "System default" to each config. It removes a corresponding fontconfig entry made by this module. "System default" is now default values for sub-pixel rendering and hinting.

Selecting "System default" also reverts Xft configs.

BUG: 105797

REVIEW: 119764
BUG: 105797