Add a transition event handler for CSS transitions

[danielghost]

Due to a limitation with how CSS transition events are handled, it isn't straight forward to identify when an element with a collection of differently configured transitions has ended. See cgkineo/adapt-list#85.

Adding functionality to core to listen to an element transition for either a named transition property, or to calculate the longest transition property, returning a Promise once finished or cancelled, will allow plugins to await as required.

[danielghost]

From testing there are a few points that may not be immediately obvious when using transitions:

• Once started, a transitions constituent properties cannot be adjusted, but they can be cancelled.
  • Changes to transition-duration mid-transition will be ignored and the previously set value will continue to be used.
  • Changing transition-property mid-transition will cancel the transition for any properties no longer included, while transition: none will cancel all transitions on that element.
  • If the value of an element property included in the transition is amended mid-transition (has to be a different value to before), that transition will be cancelled and a new instance will start. The new instance can now include an amended transition-duration (transitions for other properties not cancelled will continue to use the previous duration).
• Transitions cannot be chained. Configuring multiple element properties must take place by using the correct comma separated config, whether via the shorthand method or for each constituent property individually.