Ask which side of the confidence threshold a delete of every listed track reaches

[mattdawkins]

Replaces #1942, which put this question on the type list; it belongs on the track list.

• When every track in the track list is checked and its delete button is pressed (sidebar and bottom bar), a dialog asks whether to delete the tracks above the current confidence threshold (the listed tracks, the previous behaviour), below it (tracks of the enabled types that the list hides), or all of them. A partial selection keeps the plain confirmation.
• BaseFilterControls.annotationIdsBelowThreshold(types) selects tracks whose enabled classes all fall below threshold. Every scope deletes whole tracks through the normal removal handler; low-scoring secondary classes do not cause visible tracks to be modified.
• Specs for the scopes and the dialog gating; docs updated. The type list is unchanged.

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