_cat() does not preserve shapes

[xdevries]

The _cat() methods (subplots.jl l103) does not preserve the shapes defined on the sub_layouts, I wrote a small fix. Perhaps we can make a PR out of this? I don't know how to make a fix branch.

function handle_shapes!(big_layout, sub_layout, ix::Int)
    if !isempty(sub_layout[:shapes])
        if !haskey(big_layout,:shapes)
            big_layout[:shapes] = Shape[]
        end
        for shape in sub_layout[:shapes]
            shape[:xref] = "x$ix"
            shape[:yref] = "y$ix"
            push!(big_layout[:shapes],shape)
        end
    end
end