Fix guitar bend/dive overlap with vibrato lines

[ajuncosa]

Resolves: #34422

In this PR:

• Revert 90c0309 to ensure that vibrato lines do not ignore bends in skyline
• Fix bends/dives that cross a system break: previously such a bend was only laid out when the end note's system was laid out, so it never made it into the start system's skyline
  • Now, the start system collects its own forward bend/dive too (in systemlayout.cpp),
  • the null system2 early return (in guitarbendlayout.cpp) is relaxed, so adaptations have been made around this, and
  • the open-ended BEGIN segment from the 1st system stops reading geometry from a note on a system that may not exist yet (in guitardivelayout.cpp).