add Duplicate function to buttons and toolbars

[Matthew Law]

it'd be nice to be able to duplicate and tweak buttons and toolbars. like having a template tool bar that gets added too or having many similar preset buttons that just need an updated file path and a tweak to the color of the icon.

in all these instance it'd be great to not have to make things from scratch.

cmd+D as a shortcut would be a rad extra.

[Rafi Khan]

I've added support for Shift+Drag = Copy button. This will be in the next beta.