add a custom event called tab-selected when the selected item changes

[warnerandy]

Using the "core-select" event on paper-tabs yields 2 events (one for the unselected element and one for the newly selected element), which is unexpected. Instead I've added a "tab-selected" event that is fired when the selected item is changed (where the target element is the newly selected tab). This gives me the result I expect, one event with the newly selected tab's element in the event.

[warnerandy]

@addyosmani sorry bout the formatting, my editor is responsible for that one, I'll reset it and make my change in emacs :), and add some docs code in as well.

[addyosmani]

That looks much better, thanks!

[frankiefu]

In general, selection logic should be handled in core-selector/core-selection. If we think is a good idea to have an event just for selected then we should add it in core-selector so other elements extend from core-selector will benefit from it too. FWIW, you can tell from core-select event that it was fired for selected or unselected. If you still think it should have a separate event then I suggest to file an issue on core-selector.

[warnerandy]

Sounds fair, I was confused during the tutorial to log the event for the tabs bar and receive 2 events. I would expect two events from listening to the tab elements but not the tabs element.