This commit addresses a lot of issues, namely:
1. When the "Move left/right" action was selected on a mixer channel
menu sometimes it would segfault due to the fxline object being
deleted before it returned to a method inside itself
2. The Fader was declaring a new variable for the model whereas it
should used the inherited model from FloatModelView < AutomatableModelView
3. Some methods were doing more things than they should be and
performing unnecessary actions. A little cleanup/refactor was made
Please notice that a bug of the same class as the one referred at point 1
still exists: clicking on "Remove channel". This commit does not
addresses this issue.
Fixes#1584
