Allow advanced to search to retrieve Event custom data

Pratik - Please review and merge PR 2562 if appropriate (we'll limit the improvement to just event custom fields for now).
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We have a customer requirement to be able to search for contacts registered for events with x value as a custom field on the event. (And to make smart groups for that)

I have a patch that is working for this on our 4.4.4 deployment. While I was editing this I observed that Address, Group, Survey & Campaign fields are not accessible from advanced search. I added in custom fields for Address & Group while I was doing this & they seemed to work fine). I didn't see much sign of survey & campaign (possibly disabled) but I did enable Group & Address - although a couple of UI provisos

1) The Event custom fields are not visually differentiated from the participant custom fields
2) Selecting a Event Type does not cause event-custom-fields not relevant to this type to be hidden - not sure that happens elsewhere in advanced search either
3) There was no obvious place to put the Group Custom fields so I let it 'fall into' the custom fields pane

Also note that CRM-14157 also deals with advanced search enhancements & also has a patch in our repo