[ConfigModel] Allow programmatically adding and removing ConfigCategory
The appendCategory(ConfigCategory*) allows to add a category created by e.g. an Instantiator
that can use bindings rather than fixed arguments.
The removeCategory methods allow to remove a category when e.g. the model in an Instantiator resets.
CCBUG: 372090
Differential Revision: https://phabricator.kde.org/D7602