Add API to change data visibility

[etimberg]

This came up when removing the HTML legend. Adds two new public methods to the core Chart class for setting data & dataset visibility.

[kurkle]

Oh, I so hate long function names. It could be even clearer if called setDatasetVisibilityAtIndex 😈 (j/k)

Seriously though, one of the argument behind setVisibility vs show / hide is less functions. If that's what we are looking for, why not just setVisibility(datasetIndex, index), where you can omit the index if setting for dataset?

Another argument is the usage pattern:

setVisible(shouldItBeVisible())

vs

if (itShouldBeVisible()) {
  show();
} else {
  hide();
}

Add animations to those:

let visible = shouldItBeVisible();
setVisibility(datasetIndex, index, visible);

// need to add some parameters to limit the update to those correct elements. 
// `update` should not be a side effect of `set` function.
update(visible?'show':'hide', datasetIndex, index);

vs

if (itShouldBeVisible()) {
  show(datasetIndex, index); // update can be a side effect
} else {
  hide(datasetIndex, index);
}

And finally, I can live with the proposed ones, just wanted to share my thoughts.

[benmccann]

I like the proposed API in this PR

I still don't love the implementation of adding a hidden field and making everything check for it. I feel like it should just remove an element from _parsed and store it elsewhere to be restored rather than littering the rest of the library with if (!data.hidden) checks