Gutenberg: Gallery Block: Add Multiple Styles

[mkaz]

Add multiple styles for laying out the gallery block.
Two initial styles are Tile (default) and Masonry layout.
A potential additional style could be slideshow

See #317 and #740

[mtias]

I think slideshow should be a separate block.

[mrwweb]

Related: #783

[mtias]

We already implemented a tiled gallery design by default which is fairly flexible with cropping and columns.

[feeela]

I think slideshow should be a separate block.

I though blocks would refer to the content. A slideshow is a gallery, when speaking of the content. It is just presented in another form. In fact, there mustn't even be differences in the HTML, beside some data-* attribute to indicate, that a JS plugin should displya this gallery as a slideshow.

[mtias]

Blocks refer to what is meaningful to a user to discover and use. Different blocks can have the same HTML markup but different expectations and different UIs.