Adding option to aggregate layers, bottom to top

[adrianroth10]

Love this extension, great work with it! I was looking for a way to export the layers by aggregating them. So with three layers in an svg there are three exported images,

• one with only the bottom layer,
• one with bottom and middle layers,
• one with bottom middle and top layers.

I tried to make it work with the Use parent layers as background layers option but found no obvious option in inkscape to create the parent child connection. If there is one then I would appreciate some pointers on how and maybe a short guide can be added in the README.md. If there is not a straightforward solution for this, I did a solution by adding an option in the Layer export mode that makes aggregation possible without any parent layers. If you think it adds something to your repo I will gladly make adjustments to this commit so that it fits perfectly into your code.

Sincerely, Adrian

[adrianroth10]

Now I found how to make parent layers with the help of your test file. But still the option might be useful for people not in need of complex background schemes. It is up to you if you think it adds value.