Better attachments for broken ties & slurs
The tie and slur cases were inconsistent, and both had their problems. Slurs were using a version of Axis_group_interface::relative_group_extent that used robust_relative_extent, causing some grobs with empty extent to be mistakenly taken into account. Ties were using Axis_group_interface::staff_extent, presumably to avoid being influenced by marks or such, but that means ties from different staves could start at different points given different prefatory items at the start of the system.
Both now use a unified function that addresses these problems and also respects the convention that at the end of the system, ties and slurs stop just before the bar line, as recommended by Gould.