PIVOT-933: Allow a tree view branch listener to veto a branch expand or collapse
or defer until later, similarly to Rollup expansion changes or Accordion selection
changes.  Two new methods are added to TreeViewBranchListener to accommodate this:
previewBranchExpandedChange and branchExpandedChangeVetoed.  Implemented the
processing in TreeView.setExpanded and in the existing skin: TerraTreeViewSkin.
Add the new methods to TreeViewBranchListener.Adapter with appropriate default
processing.

Verified correct operation in a test application, and verified the Javadoc.


git-svn-id: https://svn.apache.org/repos/asf/pivot/trunk@1556748 13f79535-47bb-0310-9956-ffa450edef68