feature: Limit undo

[steff-o]

Fixes #2316 by adding the following configuration possibilities to the editor control:

Option	Description
clearUndoOnLayerChange	Boolean. When set to true the list of undoable operations is cleared when the active edit layer is changed. Default false.
clearUndoOnSessionEnd	Boolean. When set to true the list of undoable operations is cleared when pending edits are saved or aborted. Only applicable when autoSave = false. Default false.
clearUndoOnToolChange	Boolean. When set to true the list of undoable operations is cleared when editor is closed. Default false.