When installing, Spam Scores appends to the last existing filter header instead of creating a new filter header

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Describe the bug
All should be clear in the first screen shot attached. Before installing Spam Scores, my list of message headers for filters included "List-Id" and "Reply-To". After installing, the last filter header is changed to "Reply-Tox-spam-score" instead of adding an additional filter header of "x-spam-score".

Also note that other extensions alter the filter list without allowing the user to edit them (see the second screen shot, starting with "Subject (RegEx)"). I'm not sure how this is done, but you may want to consider following suit.

Reproduction steps
Install Spam Scores.

Expected behavior
When installing Spam Scores, any new filters that are added to the list of custom message header filters should be appended to the list, not to the last list element.

Screenshots

Thunderbird (please complete the following information):

• OS: macOS Monterey 12.4
• Version: 91.10.0 64-bit

Spam Score add-on (please complete the following information):

• Version: 1.4.0