FXLine: add context menu with options to remove, move & rename FX channe...

...ls

- Uses existing functionality in FxMixer & FxMixerView to manipulate channels
- Instruments sending to the manipulated channels get automatically updated
- In the future I hope to implement a drag/drop functionality instead of the clunky context menu but this is a good first step until then
- Also added in a little QWhatsThis help message for the FX line, also accessible from context menu

That lmms_math thing got mixed in accidentally, but it's also a good change: always include math.h in lmms_math - that way, other parts of the software can just #include lmms_math, and won't have to #include both math.h and lmms_math, also the yet unused sinc function in it seems to need it so this prevents problems down the line