Cocoa: Fix BackSpace keysym from Backspace to BackSpace

Mathieu Westphal (Kitware)requested to merge
mwestphal/vtk:fix_backspace_cocoa_keysym into master

On Cocoa, BackSpace used to be Backspace, it is now fixed to be coherent with Xorg and Win32.

Follow up of !10668 (merged)

Backport: release

Edited by Mathieu Westphal (Kitware)

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