Allow patterns to reference patterns in the stock patterns (from patterns.svg file)

Migrated from https://bugs.launchpad.net/inkscape/+bug/212046

Summary

If a pattern references other patterns, they are not detected in the dialog (the paint server dialog does shows them, but they aren't displayed properly, see #1363 (moved))

You would use this to have preset versions of patterns with scales applied.

Steps to reproduce:

• Find your patterns.svg file (location listed in the Preferences dialog in the systems tab, under Inkscape data (should be in the paint folder in 1.0)
• Save a backup, then replace it with patterns.svg
• Restart inkscape (or open it if it's closed)
• Create a rectangle
• Open the fill and stroke dialog and set it to pattern
• Look for Stripes 1:2 white (which has been edited to link to Stripes 1:2 Green)

What happened?

• Stripes 1:2 white is missing

What should have happened?

• Should be allowed. Note that in that case, both Stripes 1:2 white and Stripes 1:2 green should be imported into the document

Version Info:

Inkscape 1.1-dev (d3f8295e8e, 2020-07-27) Linux Mint 20