Fixed the root cause of ghost dialogs lingering after closing and blocking UI

The problem affected <spark-modal>, which extends <spark-overlay>. <spark-overlay> relies on automaticaly generated Webkit events, such as on-webkitTransitionEnd, to complete its lifecycle. It defines these handlers in its HTML definition. It turns out that such event handlers are not inherited by extending class, so the events were not delivered to instances of <spark-modal>, breaking their lifecycle. A solution was to redeclare the handlers in <spark-modal> definition.
Transition remaining dialogs from spark-overlay to spark-modal
More optimal code, no behavior changes

Instead of overriding captureHandler() from spark-overlay in spark-modal, amend it such that it's usable for both, and force autoCloseDisabled attribute on the latter (the removed override achieved the same effect).
Merge pull request #803 from ussuri/fix-ghost-dialogs

Fix ghost dialogs