Issue #10047: Show active path/layer name in dockable if only one is selected

This patch attempts to resolve #10047.

Currently, if more than one layer or path is selected in the tree view, you see "# selected items". If only one is selected, you see nothing. This patch adds code to show the active layer or path if only one item is selected. The "multi_selection_label" is renamed to just "selection_label" as a result.