Add support for multiple device and responsive design

[MikaelPorttila]

Hi,

A feature request for an already great tool: The option to design for multiple device sizes and orientations at once and then the choice to export all the needed responsive styles.

[patak-dev]

Thanks for your interest in the project! We discussed this feature a few times in the past. It would be very useful. It is not that easy to add though, but let's use this issue to track future progress.