Draft: Generate a new oxygencursors-legay binary package

With this update, src:oxygen takes over the legacy Oxygen cursors that were until now provided by src:oxygencursors. This last one could then be dropped from the archive, getting rid of a duplicated source.

Upgrade from oxygencursors (src:oxygencursors) to oxygencursors-legacy (src:oxygen) is supported.

Changelog

• Patch build system for legacy Oxygen cursors
• Call legacy cursors build recipe from parent recipe
• Generate a new oxygencursors-legay binary package
• Add extra build-dependencies on inkscape and x11-apps

TODO

• Copyright entry in debian/copyright about debian/* files should be updated. It now includes contributions fetched from src:oxygencursors debian/* files, licensed under GPL-3+ - details
• Improve debian/rules by:
  • Listing symbolic links that should be created/removed through a variable;
  • Remove the symbolic links before calling dh_clean, instead of after.
• Rename new binary package to oxygen-cursors-legacy, for better coherence with other oxygen-* binary packages - details