Move Widget's `_preAddAttrs` method override into `widget-htmlparser`.

Because `widget-base` no longer includes the ATTRS definition for `srcNode`, any attempt to instantiate `Y.Widget` when only requiring `widget-base` will fail spectacularly. Anyone using the `widget` rollup won't notice a difference.

This is related to https://github.com/yui/yui3/pull/917, and is extremely important to get in before 3.11.0 GA. http://jsfiddle.net/evocateur/EzJeQ/ demonstrates the current problem, and the test file I added verifies that the issue is fixed.