Add Phaser effect to LMMS

[LostRobotMusic]

Replaces #5168, companion PR to #5873. I already listed most of the features in the initial post, so I won't bother here.

I've fixed the bitrot issues that built up over time, and also added a few internal improvements (most notably 2x oversampling to every source of distortion). I have no idea how to build a "good" oversampling filter, but for an effect of this nature I don't think absolute perfection is required in this aspect. Just as long as most of the aliasing is removed. I just used two lowpasses each for the input and output, cutoff at exactly Nyquist of the original sample rate.

A screenshot of the UI (thanks to H4CKTON3 for the assistance)

This is a very old video so it's out of date in some aspects, but it provides an easy-access showing of some of its features:
https://www.youtube.com/watch?v=_Zyd7OcZRO8

[Monospace-V]

As per the UI, I am lead to believe AMNT is for amount.
A lot of the rest of LMMS uses AMT. Seems best to fit with convention.
Additionally, the "FRQ" dial on the top right, is that frequency? It would appear that the rest of LMMS, including a knob on your UI, uses "FREQ". I understand it may be too small a space to type "FREQ" in. Is there less space?
Can you clarify these, though? I'm not familiar with the layout of phaser effects.

[allejok96]

I know absolutely nothing about filters and such, and I haven't studied the calculations. The UI is sleek even if I don't understand what all the knobs does. This probably calls for some gitbook documentation later... All I can look at is general coding style and it looks very good. Just left a few remarks.

[allejok96]

Since EqFader.h is reused here it would be a good candidate to move to src/gui/widgets

[RainbowShatter]

how do i download this plugin?

[zonkmachine]

how do i download this plugin?

You need to build it from source. (edit: probably)

[RainbowShatter]

how do i download this plugin?

You need to build it from source. (edit: probably)

How do I do that?

[zonkmachine]

You need to build it from source. (edit: probably)

How do I do that?

1. https://github.com/LMMS/lmms/wiki/Accessing-git-repository
2. https://github.com/LMMS/lmms/wiki/Compiling
3. https://github.com/LMMS/lmms/wiki/Testing-Pull-Requests

If you're on a linux machine, this is not particularly hard.
If you're on another machine. This is a bit harder.

For future help on compiling this PR, please join and use our Discord https://discord.com/invite/3sc5su7 group.
Especially the chat room named support is where you ask questions on compiling this.

[zonkmachine]

Oopsie!