Fixed bug when serializing optional attributes and that caused unchecked optional attributes to be serialized to empty string instead of not being serialized. Note that I also fixed a bug when serializing an unchecked optional attribute inside an element, that caused the element to be discarded instead of the attribute only. Added test cases in loadSave.html autotest.

The first bug was introduced in previous commit to allow serialization of empty attributes with placeholder set to 'clear' (currently using the 'text' plugin).
Changed 'axel-editor-ready' and 'axel-content-ready' event so that they do not bubble, they are just triggered on editor's host node and on the document, bubbling made it difficult to handle multiple editors in one page. Created a special property when generating plugin DOM to prevent AXEL from shallow cloning part of the plugin presentation when they are inside a repeater, this is useful if the plugin need to reconstruct that part on each instantiation (from onInit or onAwake).
Added onGenerate to the filter mixin so that it is possible to create a filter that filters the onGenerate life cycle method.