Let's check if entries are immutable.
This lets kiosk disable the user being able to enable/disable plugins.

Matthias, Will and me have been talking about adding a new method to KPluginInfo with the signature: bool KPluginInfo::configEnabledKey(const QString &key) const, what will return if the given key is immutable or not, being it read from the proper config group (internally stored into KPluginInfo).

What do you think?