Widgets: Trigger jQuery events for widget updates.
* `widget-added` when a widget is added to a sidebar
* `widget-updated` when a widget is updated
A jQuery object of the widget is passed along to the event handler. Same events are used in the Widget Customizer, see [27909].

fixes #19675.
Built from https://develop.svn.wordpress.org/trunk@27969


git-svn-id: http://core.svn.wordpress.org/trunk@27799 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Header images: Improve accessibility of Randomize buttons.

props ehg.
see #27598, #21785.
Built from https://develop.svn.wordpress.org/trunk@27970


git-svn-id: http://core.svn.wordpress.org/trunk@27800 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Header images: Revert unnecessary `tabindex="0"` from [27947].

see #21785.
Built from https://develop.svn.wordpress.org/trunk@27971


git-svn-id: http://core.svn.wordpress.org/trunk@27801 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Update TinyMCE help for pasting content.

props chiragswadia.
see #27100.
Built from https://develop.svn.wordpress.org/trunk@27972


git-svn-id: http://core.svn.wordpress.org/trunk@27802 1a063a9b-81f0-0310-95a4-ce76da25c4cd