SPSplitView collapsible subview logic issue

[dmoagx]

SPSplitView has a logic issue with collapsible subviews:

• It doesn't take into account that more than 1 subview may be collapsible (although not at the same time)
• It doesn't take into account that a delegate can still override the collapsible-ness of a subview

Both is true for the custom query editor/results split view and as a result, when completely hiding one of them and then showing the query info pane, the UI will break, because the SPSplitView doesn't realise a subview did become collapsed.

(See #2799 (comment) )