Fix context menu not closing when placing freehand slider

[kongehund]

Fixes #31053 by overriding SliderPlacementBlueprint's DragBlocksClick to always return false.

The issue arose because osu.Framework.Input.MouseButtonEventManager did not trigger a change focus event, because the HandleButtonUp method requires that DraggedDrawable does not block clicks (due to 9482125)

[bdach]

I think this is approaching the problem from a completely wrong angle because it's not fixing what I perceive to be the actual problem.

I perceive the actual problem here to be that when you have a context menu open for a selected object, and then you deselect it via tool switching, the context menu remains open. If it were to automatically close on deselection as I perceive it should, that would prevent this slider drag scenario from ever arising in the first place.

To put it in other words: I would accept a diff that ensures context menus close on selection loss. I would not accept this diff.

@peppy would you tend to agree with that assessment?

[peppy]

Yeah, changing tools should dismiss any context menu.

[bdach]

In that case, closing and renaming issue to reflect the actual desired behaviour.