Diff Tool: Enforce consistent ordering of case structures

[theSloopJohnB]

In the current iteration of the tool, case structures are displayed with the most recently saved case on top.

This can screw up the ordering of the picture, making diffing harder. E.g. this diff:

https://raw.githubusercontent.com/DCAF-Builder/diff-pics/master/LabVIEW-DCAF/TagEditorCore/PR-351/2017-09-14/12%3A57%3A17/PLTarget%20cRIO.lvclass--CRIOUI.vi.png