Show export area preview in PNG export dialog

[Hachmann]

When exporting to png, it could be useful to show the user which area / objects will be exported.
This would make using the export dialog more intuitive, as there is a visual cue that would give a preview, which would adapt to changes made in the dialog.

For areas, a partially transparent dark overlay with a white frame, like gimp uses for its crop tool, could be used as an indicator (exported areas keep their normal color, while non-exported will have this overlay).

For selected objects, I'm not really sure what could be a good way to indicate if they will be exported or not.
I can think of making the non-exported objects not show up at all, or reducing their opacity or turning them to grey or to only show their (faint) outlines (or a combination of those).

I have no clue if this would even be possible, or if it might slow down the export dialog too much. What do you think?

[Hachmann (marenhachmann)]

Update:
doctormo made a suggestion for an improved export dialog on the mailing list. I'd prefer that to the on-canvas preview.

See https://sourceforge.net/p/inkscape/mailman/message/35674133/