Revamp snapshot history to include multiple steps

[sorenjohanson]

Proposed feature
Right now, undo/redo works on simply going back/forward one step at a time. This is awkward because even something as simple as creating one node with text involves two steps (creating the node and then the text), so removing a node effectively requires pressing undo twice.

Instead, various functions like copy paste, creating a new node with text and also hiding nodes should be summarized in one snapshot, which can then be cleanly undone or redone without steps inbetween.