My Idea is, that the Left/Right thing is exclusive to all widget the support different connections for Left an Right mouse buttons (currently only WBushButton). If we allow now to define a left button connection from a e.g. Slider, we have support it later, because we want to avoid braking old skins. This is why I feeling more comfortable to not allow Setting up Left/Right connections where it does not make sense. Checking that this is true inside the Mixxx code was one of the main tasks I have solved in this PR.