Allow back button when content has not been changed, but unlock content item

Background information

When editing content the back button is currently disabled. This is because we need to be able to always show the "Unsaved changes" dialog. There seems to be no way to show the "Unsaved changes" dialog before the back button action is executed.

Functional requirements

• Disable the backbutton when something is changed in the content item, keep the button working until then.
• When using the back button, unlock the content item. The should be possible this way: http://stackoverflow.com/questions/4945932/window-onbeforeunload-ajax-request-problem-with-chrome