Simplify Snackbar component

[unindented]

I realized it makes no sense to expose Snackbar.Text and Snackbar.Action, as MDL always expects them to be there, and they can't have user-specified children. Users should always do:

snackbarContainer.MaterialSnackbar.showSnackbar({
      message: 'Your message has been deleted.',
      timeout: 2000,
      actionHandler: handler,
      actionText: 'Undo'
});

Sorry. 😓

[developit]

@unindented what about passing text and actionText as props?

[unindented]

According to the docs, the data should be passed when calling showSnackbar. Adding them at render time doesn't seem to do anything...

[unindented]

The showSnackbar method has logic around queueing notifications, activating the element, etc., so we probably wouldn't want to bypass it: https://github.com/google/material-design-lite/blob/027e05164202d938571c358565b14528d74ed792/src/snackbar/snackbar.js#L100-L126

[developit]

Makes sense to me, though they could still be props but not in the DOM. I guess less is better here though, avoids invention.

[developit]

@unindented once refs are released in Preact proper, might be good for us to provide an alias for show() -> showSnackbar() on the component.

[unindented]

Tru.