[TIMOB-17828] Declare constants for event name strings

Using strings for event names is prone to typos which cause silent failures. Declaring constants, much like we have for style enumerations, would resolve this problem. e.g.

$.myButton.addEventListener('cilck', function(e){
    // nothing happens but its not obvious why
});

Could be replaced with:

$.myButton.addEventListener(Titanium.UI.Button.CLICK, function(e){
    // ...
});