Open the gallery from placeholder in the initial creation state

[westonruter]

Adds the initial multi-select mode without having to shift-click to select multiple images, as well as the “interstitial” state to re-order the images as desired. This adds functional parity with the classic post editor.

Props @obenland for his work on Gallery widget which was adapted into this PR: xwp/wp-core-media-widgets#120

Fixes #1401.

[coveralls]

Coverage decreased (-0.06%) to 17.752% when pulling e890db3 on add/create-gallery-state into 4564ee0 on master.

[mkaz]

@westonruter - tested out and reviewed, this one looks good and probably better since it integrates with MediaUploadButton, so we should use this instead of #1940

One thing you might want to add is something along the lines of this slimImageObjects function which attempts to reduce how data getting saved as attributes

cc: @youknowriad

[westonruter]

@mkaz to be wrapped around the toJSON calls in the following?

		if ( multiple ) {
			onSelect( selectedImages.models.map( ( model ) => model.toJSON() ) );
		} else {
			onSelect( selectedImages.models[ 0 ].toJSON() );
		}

[mkaz]

@westonruter yeh, I think that'd be a good spot to add it 👍

[westonruter]

@mkaz I'll cherry-pick your relevant commit(s) into this branch.

[mkaz]

@westonruter I can help out, see a2d99ad which adds the function to the branch.

[westonruter]

Instead of having both slimImageObjects and slimImageObjects, how about only having the former? If you just have slimImageObject then we can just do:

if ( multiple ) {
	onSelect( selectedImages.models.map( ( model ) => slimImageObject ( model.toJSON() ) ) );
} else {
	onSelect( slimImageObject( selectedImages.models[ 0 ].toJSON() ) );
}

I guess I just don't see how having a separate slimImageObjects function is necessary.

[mkaz]

heh, that is better - I think because I started with the first method, I kept it around.

[westonruter]

Humm. Build is failing:

phpenv global 5.2
rbenv: version `5.2' not installed

I'm going to rebase against master and see if that fixes it, since I know there were some Travis changes in the past couple days.

[westonruter]

Rebased. Former HEAD was f5df81e4.

[aduth]

This assumes a dependency on Underscore which we don't explicitly define. It's only incidentally not causing catastrophic failure.