Fix VirtualisedListContainer rows not returning child to pool on disposal

[bdach]

CompositeDrawable.Dispose() doesn't do anything to its children except dispose them, which is problematic, since it leads to two things happening in the context of VirtualisedListContainer:

• The pooled child is never returned to pool (because returning happens automagically on parent change, which doesn't happen here)
• However, the pool continues to retain the reference to a disposed pooled drawable, which means that it (a) keeps drawables that will blow up if they were ever to be used, which isn't possible because (b) these drawables are never returned, which means that the pool will inevitably start overflowing.

Thus, aggressively remove the pooled child at the start of disposal to counteract this happening.

Can be semi-manually tested using 0b7106b:

• Open test scene
• Open performance tool window (ctrl-f2)
• Scroll to the part that shows active / total pool capacity
• Spam the added "replace items" test step
• Watch the pool begin to overflow.