DTGOV-4 Deployment target configuration/management UI

Handlers should be registered with the specific widget that fires the event you want to catch.  So this line should be something like:

_targetsTable.addDeleteTargetHandler(handlerDeleteTarget)

Also note that the work to actually delete the target should be done in the Page, not in the table widget.  The table widget should simply fire the "Delete Target" event.  This event should include the target that was deleted.  The page will then capture that event, prompt the user with a confirmation dialog, and then do the RPC call to delete the target, then finally refreshing the table.  The table itself should never do this kind of work.

For an example of eventing in GWT see this:

https://github.com/EricWittmann/apiman/blob/master/apiman-dt/apiman-dt-ui/apiman-dt-ui-war/src/main/java/org/overlord/apiman/dt/ui/client/local/events/BreakContractEvent.java

https://github.com/EricWittmann/apiman/blob/master/apiman-dt/apiman-dt-ui/apiman-dt-ui-war/src/main/java/org/overlord/apiman/dt/ui/client/local/pages/AppContractsPage.java#L79-L84

https://github.com/EricWittmann/apiman/blob/master/apiman-dt/apiman-dt-ui/apiman-dt-ui-war/src/main/java/org/overlord/apiman/dt/ui/client/local/pages/AppContractsPage.java#L167-L180