Feature 8558: allow preservation of menuindex and aliases when duplicating context.

Changes are:
- add checkboxes on the duplicate context window, with two options for preserve aliases and menuindex (selected by default)
- changed modresource to take into consideration the two new option
- change the context duplicate processor to pass the options
- add the two strings in the English lexicon 
See http://tracker.modx.com/issues/8558 http://tracker.modx.com/issues/8195 for associated bug/feature request.