Centralizing the plugins' config will ensure that all the plugins store it
the same way (ie: not in random files scattered around, like until now).
The base KCM class, together with the already existing base plugin class,
will give easy access to all the plugins to this centralized config. Also,
now the settings are not shared across devices (that is: every device can
have different config for a same plugin).
Note: This commit requires KCMUtils 5.9
REVIEW: 122927
