flag to allow onMouseClick during a drag

[p0nce]

Currently: always allowed! if dragging with some mouse button, another onMouseClick could come from another mouse button.
For every widget in existence.
Most widgets are then only "correct" because the host eat up the nested begin/endParamEdit, and stich together adjacent periods of begin/endParamEdit.

Most of the time, this is only dealt poorly with regards to parameters being dragged.

However removing this is super tricky because of double-click, in some OS double-click comes after single-click and this must be accounted for.
This would also be a breaking change unfortunately.