add widget for wait time

[mpadge]

When I load the data for New York City, and click on the menu, it takes a few seconds to load, during which time the main screen looks like it normally does. This has the effect of making it seem - for those brief few seconds - like my selection has done nothing. It'd be good to add a spinny widget that loaded as soon as something is clicked, to give immediate visual feedback that something is happening.

[Robinlovelace]

Closing as not in scope for now, can re-open at a later date.