spark-dialog fixes

TBR @devoncarew 

1. Enable external sizing of spark-dialog: previously, if a non-0 height was set on a spark-dialog, it would render a transparent element blocking everything under it.
1. Make body `flex`, so it will auto-size itself to the dialog's size when it is set externally.
1. Also make spark-dialog's body `display: flex`, so it will auto-fit any distributed light DOM children that are `flex`.
1. Also gave better names to events emitted by spark-overlay.

The above amounts to easy-to-do dynamically sized dialogs with auto-fitting contents.