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.