Added
* `add_child_at_mpl_point()` function to simplify needing to separately call three
different functions to add a child, get pixel converted mpl data locs, and
then connect the new child
* `display_output()` function to support creating an ipywidgets `Output` and
rendering any displayable such as a static matplotlib figure with a single call
Fixed
* Missing top level module imports
* Connection to matplotlib ipympl canvas not always using the instance within the
container