CMS editor history actions.

[Pavel910]

Sometimes when undoing/redoing, elements get messed up especially when manipulating row columns. A possible cause is the fact that when manipulating columns multiple actions are dispatched during a single "manipulation", thus making history middleware create states that are incomplete.

Things to inspect:

• see which element plugins dispatch multiple actions during one transaction
• try refactoring code to make row/column elements more dependent on each other but handle the entire logic in the Row plugin to avoid dispatching multiple actions.
• if all else fails, try adding a commit changes action that will trigger history middleware (maybe this is the best way to begin with?) (this doesn't work)

Implementation

Delete/Drop logic must work with a single action dispatch, meaning, once an element is dropped or deleted - plugins must modify the parent element and return the new data which will then be set using update element action at once. That way we avoid multiple history triggers.