Node conditions should be in the "Content" category

Node events and node actions have always been in the "Content" category - this is because the entity label (=Content in the case of node entities) is used to define the category.

However, Node *conditions* currently appear in the "Node" category - this is a legacy of D7.

This patch moves all the Node conditions into the "Content" category, for consistency with events and actions.

The UI is much easier to use if all node operations appear in the same category and if we use the same terminology throughout the UI.