Exported file name should be saved in svg as relative path; current behavior 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
issue