fix: multidragkey not working as stated in docs

[maxbol]

According to the docs: The multiDragKey is "a custom keyboard key may be designated as having to be down for the user to be able to select multi-drag elements."

At the moment, MultiDrag allows user to select a single element even if multiDragKey is not down, if set. With this change, mutliDragKey has to be down to select even one element.

[jjeff]

This is great! Makes much more sense now.