More system object refinements

[mike-spa]

Resolves:

• Layout breaks still showing in the system markings list.
• When staves are hidden, system objects should fold downwards (i.e. the upper ones should replace the lower ones), but they were currently doing the opposite
• Remove the system object UI from part scores. At the moment the UI doesn't work for part scores because the whole tree model that makes up the instrument panel referes to the master score (cause that's where the full list of instrument and staves is) but this doesn't really work for system objects. It's not hard to fix, but we'll do it later. @RomanPudashkin