Seems the string "Trash Applet" is used for the thing that can be added to the panel. But should it be "Trash Plugin" instead? The other items that you can add to a panel is either called plugins or nothing.
I don't like the applet term, but renaming "tpa" to "tpp" is may cause regressions and will be confusing for users and packagers. Changing only the strings will cause a UI/code naming mismatch which is confusing for maintainers and contributors.