Make `sendAction` available from TargetActionSupport.

`sendAction` is essentially just a nicer API around
`TargetActionSupport`'s `triggerAction`. Currently, it is not possible to use
`sendAction` from anything that is not an `Ember.Component`.

`Ember.TargetActionSupport` is currently only used by `Ember.Component` and
`Ember.ViewTargetActionSupport` (which itself is unused internally). This
change just moves `sendAction` up into the mixin where it can be reused by
devs needing these same semantics.