By removing the postfix we consistently use, we get exactly the same string as before
However, all our pluginId logic is now done as part of the buildsystem.
Before, they were in the JSON metadata, the buildsystem and the string constructor parameter
KPluginFactory has the KPluginMetaData object in any case, do using it
doesn't create any overhead.
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
