Fix action slot dropdown missing labels

When adding a button in Designer when clicking in the action slot dropdown there are some labels missing in it.

how to reproduce:

• in studio go to Designer
• in the UI tab open Buttons
• hover the + icon in the bottom right of the screen and then click in the Button icon
• go to the Slot field and click to open the dropdown >> some fields are missing the labels - bug

commit and ticket where the bug was introduced:

https://github.com/nuxeo/nos/commit/97f7f289c89659d71185bd4b23ba21f80d03ab4a 

NXS-6426