Create screenshots from plugin prefs

This adds the plugin prefs using our plugin-prefs tool to the screenshot CI tests.

Draft as we trigger #865 (closed) which is nice so we can fix it before merging (it passes locally here as it's a race when opening the overview). We also need !1180 (merged) as a base and need to fix the locale dir so prefs plugin translations get picked up correctly.

@kop3161 adding !1170 (merged) prefs is then just a 3 liner.