New widget: Piano keyboard widget

A piano keyboard widget to be used for audio and music theory stuff.

Features:

Multi-octave piano keyboard.
Keys spanning arbitrary ranges of notes.
Using correct key metrics.
Rendering pressed piano keys with pressure support.
Key markers to show chords and scales.
Key labels to show custom labels (note names, FT2 style instrument-sample assignment, etc.).
Acting either as an input or an output widget.
Key IDs following MIDI.

Screenshot:

20220629_161728 (1)

xTibor / egui_extras_xt