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Add option to Copy the active view layer, and add an empty view layer

Modify the view layer add operator (and underlying `BKE_view_layer_add`)
to allow for copying the current view layer, as well as adding a new one
but with all LayerCollections disabled by default (this is important for
heavy scenes where currently adding view layers can take a long time due
to enabling every collection by default).

Reviewed By: mont29

Differential Revision: https://developer.blender.org/D6862