flag to allow onMouseClick during a drag

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.

Start by tracing the widget paths for onMouseClick, dragging, and nested begin/endParamEdit calls. Check how double-click ordering differs across operating systems before deciding the flag’s behavior; done means the interaction is consistent for every widget without breaking legitimate double-click handling or parameter-edit periods.

Written by the indexing model from the issue text.