Set the id of widgets after loading them from the xml

This change is small but useful. This pull will set the id of widgets after loading them from the xml, if the id was specified in there. It's not really useful to have a null-id, if the id was not-null in the xml.

This allows us to use the id in a getEvent-handler of a FlxUIState. For example, different actions for multiple buttons can be handled without adding a <param>-tag by just setting the id.

I'm proposing this, because managing <param>-Tags seems unnecessary in busy layouts. Maybe I'm missing something?