When using WA.state, using `"myVariable" in WA.state` would always return false.
This is now fixed by adding a "has" method on the Proxy class.
Also, added a `WA.state.hasVariable` method.
Setting a variable would makes the application enter in an infinite loop of events (between all the scripts and the back)
This fix makes sure a variable does not emit any event if it is changed to a value it already has.
