Display tick marks on iOS Slider

iOS is the only backend where the native widget has no built-in support for displaying tick marks. However, it may not be too difficult to add them: see https://stackoverflow.com/questions/42898935.

Locate the iOS Slider backend and compare its native widget capabilities with the linked Stack Overflow discussion. Done means the iOS slider displays tick marks consistently with the other backends, with the relevant existing checks or tests updated if available.

Written by the indexing model from the issue text.