Add visual aid to indicate when some images lists are scrollable

[mnalis]

Use case It is very undiscoverable that some images are scrollable, thus that information is missed even by long-time users and developers, see e.g. https://github.com/streetcomplete/StreetComplete/issues/6233#issuecomment-2845497103 and https://github.com/streetcomplete/StreetComplete/issues/6233#issuecomment-2847877471

This applies to images to in the infobox, but also potentially other places (like recently used lists) like mentioned e.g. in https://github.com/streetcomplete/StreetComplete/pull/6022#issuecomment-2544007529

Proposed Solution

Visually indicate in some way that those image/icon lists are scrollable. Perhaps a fadeout, or a border around a picture that remains unfinished, or drawing an arrow, or something else?

quick mockups:

(Those mockups are just to demonstrate the idea, I'm obviously no graphic designer)

[danieldegroot2]

( See also MapComplete/issues/2390 for example from another application )

[westnordost]

Implemented for the last-picked buttons in building levels and list pickers #6490