Add new hook to the top of the widgets admin page

[ptahdunbar]

There is a hook at the bottom of the widgets admin page, but not at the top before the "available widgets" section. It'll be useful to have one there so users won't have to scroll down the page to access features added by plugins.

[ptahdunbar]

Adds a new 'widgets_admin_page' hook above the "available widgets" section.

[nacin]

[13525]

[nacin]

(In [13525]) Add action to top of widgets.php. props ptahdunbar, fixes #12190