Implement #14209-2: Allow setting line hook heights to negative values

[Gravifer]

Resolves: #14209-2 #15871

Text lines under staves may be expected to have hooks pointing up, as the behavior of built-in lines, e.g., the pedal line. User should be allowed to set hook height to negative values in the Properties tab.

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[Gravifer]

An itsy bitsy change, I don't think this would break anything. I hope this PR of minimal effort does not seem distasteful.

[cbjeukendrup]

@bkunda or @oktophonie Could you please test this briefly?