Refactor Input boost to be easily ifdef'd out

**What is the current behaviour and why should it be changed?**

The input boost feature is currently coupled too strong to the client. This should be refactored with the goal to be able to compile Jamulus without it.

**Describe possible approaches**

Introduce a class similar to the Reverb class that applies input boost.

Suggestion would be an architecture that allows setting the gain, getting the gain and applying the gain to a channel. We could abstract this in a general "ClientEffect" or similar class which could also handle the "pan".

**Has this feature been discussed and generally agreed?**

No. This issue should serve as defining the spec.

Cc: @pljones @dingodoppelt

Start by reviewing the client-side input boost implementation and the existing Reverb class, which the issue identifies as a possible model. Clarify the architecture and scope in the issue discussion before changing code. Done means input boost can be compiled out while its gain can be set, read, and applied to a channel.

Written by the indexing model from the issue text.