make move/duplicate tabs with mouse controllable

[md-work]

Problem:

Currently tabs are always duplicated when dragging them with the mouse to another Atom window.

But when mouse-dragging tabs to another pane, they are always moved (without duplicating them).

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Possible solutions:

I suggest to make this controllable, by pressing the CTRL key while mouse-dragging a tab. A widely spread behavior for the CTRL key in this scenario would be, to move a tab when CTRL is not pressed and to duplicate a tab when CTRL is pressed. Thats for example how Firefox handles it.

Another alternative would be, to always move and never duplicate a tab when mouse-dragging it. And for duplicating a tab you explicitly need to right-click on the tab and choose "duplicate tab". That's for example how Google Chrome handles it.