add _WORK layer and ignore its content on export

[Tosyk]

could you please add a _WORK layer the content of which will be ignored while the export?
that would help to keep all the WIP details within a single file :)

[pudingus]

I can see this being useful, I've had the need for this in the past too. I picture it like this: you can choose which layers you want to export, or export only selected objects, or choose a Selection Set