Ensure Gradle menu is available when contributing menu items

This fixes the menu extension discussed here: https://github.com/spring-projects/eclipse-integration-gradle/pull/66

The deprecated <objectContribution> actions are funny in that you can't guarantee the order at which they will load, so in this case if eclipse reads the 2nd <objectContribution> with the actions before it reads the 1st <objectContribution> with the menu it will complain "invalid path", but eventually the menu is added so things seem to work out (that is why the menu can be seen at all I guess).

But this fix simply moves all of the actions for the menu into the same <objectContribution> as the menu itself.