Add multi-select support to custom tree view

[alexr00]

[alexr00]

Not all trees will want to support multiselect. We should also add API for this. I propose we add a canSelectMany to the existing TreeViewOptions:

	/**
	 * Options for creating a [TreeView](#TreeView)
	 */
	export interface TreeViewOptions<T> {

		/**
		 * A data provider that provides tree data.
		 */
		treeDataProvider: TreeDataProvider<T>;

		/**
		 * Whether to show collapse all action or not.
		 */
		showCollapseAll?: boolean;

                /**
		 * Whether the tree supports multi-select.
		 */
		canSelectMany?: boolean;
	}