System markings UI refinements

[oktophonie]

Two small matters, first:

• Create new score
• Add a system break

'Layout break' becomes listed as a system marking type at the top instance, but there is no need for it to be (they are purely formatting elements). (Fix actually in #26572)

Secondly: clicking an instance of system markings in the Layout panel will change it to 'System objects hidden', but the eye icon does not change to closed until clicking it a second time. A third click will restore all to visible:

20250217-0837-26.4120959.mp4

This requires there to be some type(s) of system markings present in the score.

[Tantacrul]

• Layout breaks certainly should not be considered system objects and (I imagine to your relief) we didn't specify that they should be in the design! This one is definitely a bug.

[sammik]

Scale with staff size doesnt work. (But it works for Time signatures) EDIT: ups, sorry, already reported #26551

• If drag below staff, and click property "above", it remains below. (Oposite works as expected)

System_object_drag_below.mp4