Allow GUI editing of Event Intro layout HEADINGS

Currently you are able to choose which fields are visible when listing events. This is editable via

Components > RSEvents! > Settings > Events > Table Layout

However once you have added/removed items the headings for the changed items are not reflected as these are hardcoded. This requires PHP code editing which is not upgrade-safe.

We would like RSEvents! to intelligently look at which fields are being listed and set their column headings appropriately.