Some random notes now that grid editor layout has landed on master:

• should we deprecate ViewColumn in favour of new API that exposes the grid system to extension authors?
• if we decide to keep ViewColumn, how should it map to the grid system
  • should the order be by creation time (current solution)
  • should the order be by visual appearance (this would probably map best to our previous behaviour)
• should we just introduce an OR-type for ViewColumn that is number so that the column is never undefined when > 3? in the current state viewcolumn will be undefined for any editor that opens in a position > 3 which is bad because extensions might have used this as a check to find out if the editor is in the editor area or not (e.g. panel, embedded editor)
• we probably want to introduce a new dynamic ViewColumn that allows to open an editor to the side of the active one (ViewColumn.SIDE_BY_SIDE?) so that an extension can simply use that instead of having to make the math with absolute view columns