[HxSidebar] Clicking non-expandable node when MultipleItemsExpansion should collapse current node

[hakenr]

The current implementation uses Bootstrap's collapse parent, where setting the parent makes the collapse behave like an accordion (collapsing the other sections under the same parent). However, this doesn't handle scenarios where some nodes aren't expandable (no child nodes). Navigating to such a standalone node should probably cause the currently expanded node to collapse.

cc @alexax578

[hakenr]

For our current PWT project, we will resolve this in derived component. To support such customization, we need to expose (protected virtual) the HandleCollapseShown and HandleCollapseHidden methods (together with already exposed protected bool expanded field).