Move structure in gui

Open username-removed-256065 requested to merge stly/ase:move-gui-structure into master

In ase-gui: Allow the entire structure to be moved using shift + right mouse button (and then move the mouse)

It contains an odd line to determine from a tkinter event that shift is pressed. Therefore it needs to be tested on Mac also.

Update: It still has the strange behavior on Mac, where the structure moves much more than the mouse is moved. This will be fixed eventually.

Edited by username-removed-256065