Restoring of cache settings (SettingsCache::loadToSettings) is not reliable

[tsujan]

SettingsCache::loadToSettings() restores the current values but if new keys have been added to the setting, it couldn't remove them.

Because of the internal cache used by Qt itself, using QSettings::clear() at the beginning of SettingsCache::loadToSettings() won't have any effect, even if QSettings::sync() comes after it.

I've encountered the problems caused by Qt's internal cache in one of my apps; they are very annoying. But GLib's Key-value file parser is completely reliable. So, I propose that we use it instead of QSettings in SettingsCache.

Context: I encountered the problem while fixing lxqt/lxqt#1513 (the reset button).