[Request]: Use button row list add pattern

[jamiethecat]

Describe the request

Instead of a header suffix, consider using a placeholder button row in an empty state, and have a button row at the end of the list to add more items. From what I've seen, this is relevant for the Keywords preferences group in the preferences, and the Keywords and Phrases preferences group inside the New Filter dialog.

Implementation Details

• [X] This follows the GNOME HIG.