dnd5e: Switch Tidy Sheets to "One Add Button"

[kgar]

Currently, Tidy uses the per-section Add button:

Because of changes from dnd5e 4.0, some inline add button scenarios are no longer feasible, such as adding a Passive vs. Active feature ability in the Features tab. Creating a new item that is delineated as active implies also creating an Activity under that item. Which Activity should be created is not something Tidy can reasonably determine. For this reason, it would be best to move to the "One Add Button" pattern instead: