ld-analyse feature request: VBI editing

[Gamnn]

There are times when it would be useful to manually edit a frame's VBI, such as when unusual VBI corruption confuses ld-discmap as in #466. Adding custom chapters or stop codes could be another use case.

Adding a mode to the VBI window in ld-discmap where it unlocks the fields for editing, then encodes the changes and saves it to the json when saved might be one way to do it. When unlocked, it should have buttons both for saving, and for discarding the changes.