frontend: merge joystick profiles and interface profiles

[ES-Alexander]

Current behaviour

We have independent "Profiles" available for the interface vs the joystick, which is confusing and seems unnecessary.

Expected or desired behaviour

Remove the concept of separate joystick Profiles, and just tie a given joystick button function mapping set to the interface Profile.

A separate mapping per joystick type can still be stored within one Profile, but if a user wants separate joystick mappings for the same device type (e.g. 2 mappings for a PS4 controller) then they can create a separate Profile (or User) for the extra one(s).

That seems like a logically simpler structure than the current approach, and is easier to explain, understand, and use.

Prerequisites

• I have checked to make sure that a similar request has not already been filed or fixed.