Pad: Add Pop'n Music controller type

[RedPanda4552]

Description of Changes

Adds an implementation of the Pop'n Music controller. Yep, that's the whole thing.

Side note: Like with guitars, automatic mapping with this controller type will make no damn sense at all because it isn't a normal gamepad. That is not a bug, that is a consequence of having a unique button layout, and it is up to the user to do it manually if they want mappings that make sense. Get over it.

Rationale behind Changes

Easier for users to set up than trying to guess what DS2 controls they need to map to for a Pop'n controller to make "sense". At least some of the games don't strictly require the Pop'n Music controller, but perhaps others might, so that's another reason.

Suggested Testing Steps

Fire up one of the Pop'n Music games, see if the controller reacts the way you expect it to.

[RedPanda4552]

As per usual, I am no artist, and I have suppled a scuffed png for the reference image in the mapping screen. Please hold merge until we can get a proper svg for it.

[RedDevilus]

How does this supposed eldritch horror draft of a png look like? ;)

[RedPanda4552]

Another point of interest is that we do not have font icons directly suitable for these buttons, since they are all circles with solid colors. I went ahead and just used the DS2 button icons which would match the literal bitmask that the Pop'n controller uses.

I am open to changing that, but someone would need to find a way to supply icons which can somehow convey both position and color of the buttons, in a tiny and monochromatic font icon that will render as text at the bottom of the OSD. Not exactly something I think there's a better solution for, but I hope I can be proven wrong on this one.

[stenzek]

Looks fine from a quick glance