Move to the next track part with PageUp/PageDown keys

[maiko3tattun]

New Feature

Use shortcut keys in the pianoroll window to move to the part of the track above or below in the same position.

2025-02-09.193220.mp4

This feature does not change the horizontal scroll position of the pianoroll window, and the criterion for whether the part is in the same position is the center of the notes canvas.

Bug Fixes

• Since voiceParts and waveParts in UProject are explicitly assigned null, I changed them to nullable types to avoid confusion.
• Fixed no scrolling when using End key to move play position.