WIP: Add extension events for the main relation widgets

[mjauvin]

Add the following events for the primary RelationController widgets:

- relation.extendViewWidget
- relation.extendManageWidget
- relation.extendPivotWidget

Questions:

• do we need the global events?
• do we need the following events:

   - relation.extendViewFilterWidget
   - relation.extendManageFilterWidget
   - relation.extendRefreshResults
   - relation.extendConfig

Note:

I didn't add parenthesis around the arguments in the handler method signatures because I find this is a pain when copy/pasting an example to get you started as they need to be removed.

I think we should do the same for the other events examples elsewhere in the codebase.

[mjauvin]

Any feedback on this welcome.