Make UI patterns conversion to slot alterable

Problem/Motivation

In ui patterns, there is a conversion mechanism which is used automatically to allow prop types to be converted,
and a chain converion is totally possible, like : int -> string -> slot.

so you define an ui patterns source which returns an int, you declare it is compatible with a prop type integer, and it becomes available for strings, slots..etc magic :)
the conversion is made directly in the target prop type.

this is not well alterable from the exterior, especially when new prop types are defined by other modules (like ui_icons).
The most natural way of using that internal "framework", is to allow some new prop types to be converted to slot, relying on the convert mechanism instead of having to declare slot as a possible prop type and managing this in code, where it could be handled automatically, only using a small convert code at the right place.