[structure view] keep expanded/collapsed state of tree nodes when updating the tree

When the structure tree view gets updated (for whatever reason), the expanded/collapsed state of the nodes should not be changed. At the moment, all nodes are expanded by default - and this default state is re-created when the tree gets updated (e.g. by the user changing a file). We need to keep the expanded/collapsed state somehow and not change it every time the tree gets updated.