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

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.