Let \alterBroken really support callbacks

The regtest had a test case for callbacks in the list of property values passed to \alterBroken, but the support was actually accidental. What happened is that on the first time the property was looked up, the callback generated by \alterBroken returned the callback given by the user, which would appear as the value of the property. On a second lookup, that user callback would be resolved. This led to inconsistencies (for example, in the regtest, point-and-click didn't work on the hairpin, because its extents used the wrongly computed value). This lets value-for-spanner-piece execute the value as a callback if it's a procedure. Support for unpure/pure containers is also added, which is especially useful with the grob-transformer.