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
Benefits:
- We had config files in different paths, now they will be centralized.
- Daemon, LanLinkProvider and NetworkPackage depend only on Qt now.
- KConfig is accessed with strings as keys, where a typo can go unnoticed.
- Daemon is now thinner, as it doesn't need to initialize config values.
- QCA::Initializer is in a single place now (was in Daemon and Plugin).
