Improve/normalize some attribute names
spark-overlay: specify animation via dedicated attribute, not CSS class
spark-overlay: switch to a less error-prone event attachment mechanism

Attaching via `on-<event>` mechanism was error-prone:
- Event names can be misspelled
- Handler names can be misspelled
- Attached handlers are not inherited by extending elements (such as spark-modal here)
Merge pull request #1338 from ussuri/improvements-in-selector-and-overlay

Improvements in spark-selector and spark-overlay