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
This meant to add a lot of dependencies to each plugin since we had
KDELibs4support as PUBLIC link meaning that anything linking against
kdeconnectcore was linking at the same time to mostly all frameworks.
Now each plugin has more or less its dependencies in the CMake some
still depend on KDELibs4Support.
For the mousepad plugin I needed to add a fixX11.h file that basically
undefines/defines again some stuff xlib has that conflcits with normal
C++ and Qt.
Before it was not conflicting because some lib within KDELibs4Support
was including this file, but now we have to do it ourselves.
