Exported file name should be saved in svg as relative path; may leak private system information

Migrated From: https://bugs.launchpad.net/inkscape/+bug/1492284

Steps to reproduce:

• open Inkscape
• open svg file
• select one of the shapes
• export the selected area to PNG in a directory near (but not the same as) the svg file
• Save the SVG

What happened?

The full path of the png is saved into the selected shape's svg.

Sub Issue: It's not overwritten or cleared if an export is done to a file with the same directory. Sub Issue: The filename isn't saved at all if it's the same directory.

What should have happened?

It should save the relative path always, unless the path is very far away (more than three parent jumps (/../), then it can choose to save the full path instead.

Inkscape Version and Operating System:

• Inkscape Version: 1.0 Alpha
• Operating System: Ubuntu
• Operating System version: 18.04