Add usage statistics for VE

We need to implement the stats for the following (I've included suggested names for the events):

• Edit content button
  • CMSChannelsEditContent
• Saving a document in visual editing
  • CMSChannelsSaveDocument
• Switch to content editor button
  • CMSChannelsContentEditor
• Publish button in Channel Manager (sends user to Content perspective)
  • CMSChannelsContentPublish
• Switch to full screen button
  • CMSChannelsFullScreen