* 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.
git-svn-id: https://develop.svn.wordpress.org/trunk@41590 602fd350-edb4-49c9-b593-d223f7449a82
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