GP import: extract chord-diagram fingerings (GP5/GP8) + GP8 chord names

[byrongamatos]

GP chord diagrams contain per-string fingerings and GP8 carries chord names, but both converters drop them (fingers=[-1]; GP8 name='').

Work

• lib/gp2rs.py (GP5) + lib/gp2rs_gpx.py (GP8): extract fingerings from GP chord diagrams; GP8: extract chord names.
• Imports then arrive with rich templates the editor now preserves.

Non-blocking; lands after the editor chord-model foundation (got-feedBack/feedBack-plugin-editor#3) so the data isn't immediately wiped. Part of #334.