make more operations undo-able

[radiatoryang]

to get undo support working, Merino uses a secret temporary ScriptableObject (MerinoTreeData serializedTreeData) so that it can use Undo.RecordObject() with it... but I think I don't quite understand the undo system well enough to fix various bugs / extend the functionality

Merino also keeps its own stack of undo snapshots, so that it can compare those snapshots to Unity's undo stack, diff strings, and move the keyboard caret cursor back to the undo point (see MerinoEditorWindow.OnUndo())

we still need undo support for:

• drag and drop in TreeView... reordering tree (maybe hook into OnTreeChanged()?)
• AddNewNode()
• DeleteNodes() (this is tricky because I also don't understand the undo system too well, and I suspect you also have to revert the currentFiles and fileToNodeID dictionary as well... should we store this data in serializedTreeData somehow?)

I suspect there's probably some much more elegant way to do all of this, instead of manually recording undo snapshots everywhere in all these different methods... again, help would be appreciated from someone who understands Unity's undo system better, and can explain more about how to handle all this