Widgets: Add gallery widget

[westonruter]

As media widgets were introduced in #32417 and an image widget in #39993, there should also be a Gallery widget as outlined at ​https://github.com/xwp/wp-core-media-widgets/issues/62

Pull request with implementation in Core Media Widgets feature plugin: ​https://github.com/xwp/wp-core-media-widgets/pull/120

GitHub project for gallery: ​https://github.com/xwp/wp-core-media-widgets/projects/4

Gallery widget now available in the Core Media Widgets v0.2.0 feature plugin. Please test to support WP4.9 merge: ​https://wordpress.org/plugins/wp-core-media-widgets/

Report any issues at ​https://github.com/xwp/wp-core-media-widgets/issues

Bumping priority to high for visibility and alignment with 4.9 goals, and given proximity to beta 1 deadline.

Issues reported for Gallery widget:

• ​190: Columns in gallery widget preview do not match the frontend
• ​191: Ensure core themes have styling compatibilities for gallery widgets

​https://github.com/xwp/wordpress-develop/pull/265

​https://github.com/xwp/wordpress-develop/pull/265/files/9fca87f..2562ac1

@joemcgill FYI, see 41914.1.diff​ for a ​delta of additional changes I've made to the gallery widget in the core patch.

​https://github.com/xwp/wordpress-develop/pull/265/files/2562ac1..79890bd

Gallery widget populated with images

Selecting images for adding to gallery widget

Editing images that had been selected for gallery

Gallery widget empty in initial state

In 41590:

Widgets: Introduce Gallery widget for displaying image galleries.

• Galleries are managed in the widget in the same way they are managed in the post editor, both using the media manager.
• Gallery widget is merged from the Core Media Widgets v0.2.0 feature plugin and it extends WP_Widget_Media in the same way as is done for image, audio, and video widgets.
• Model syncing logic is updated to support booleans and arrays (of integers).
• Placeholder areas in media widgets are now clickable shortcuts for selecting media.
• Image widget placeholder is updated to match gallery widget where clicking preview is shortcut for editing media.

Props westonruter, joemcgill, timmydcrawford, m1tk00, obenland, melchoyce.
See #32417.
Fixes #41914.

Follow-up: #41969 (Ensure Gallery widget is styled properly across widget areas in bundled themes)

Jetpack support issue: ​https://github.com/Automattic/jetpack/issues/7845

Small issue I noticed when I was testing the gallery widget. I opened Gallery widget in Customizer and changed only image caption, it does not refresh the preview.

Replying to rabmalin:

Small issue I noticed when I was testing the gallery widget. I opened Gallery widget in Customizer and changed only image caption, it does not refresh the preview.

@rabmalin thanks, good catch. I opened a defect in #41979.

Dev note: ​https://make.wordpress.org/core/2017/09/25/introducing-the-gallery-widget/

In 41772:

Widgets: Remove "Replace Gallery" button from Gallery widget since redundant with "Edit Gallery".

Props Presskopp, juhise, melchoyce.
See #41914.
Fixes #41994.

In 41840:

Widgets: Update default link_type for Gallery widget from none to post for consistency with wp.media.galleryDefaults.

Props bor0.
See #41914.
Fixes #41985.

In 41872:

Widgets: Update preview for Gallery widget when one of its attachments is modified in the media modal, outside the customized state.

• Ensure that changes to captions are shown in preview when modified in media modal.
• Also keep wp.customize.widgetsPreview.renderedWidgets updated when widgets are added or removed.

See #41914, #37887, #40403.
Fixes #41979.

In 41951:

Widgets: Improve extensibility of the Gallery widget and of media widgets generally.

• Introduce a widget_{$id_base}_instance_schema filter for plugins to add new properties to a media widget's instance schema.
• Pass all of a gallery widget's instance props to the gallery media frame, not just the ones that core supports.

See #32417, #41914.
Fixes #42285.