Accessibility: Widgets: Further improve spacing between Widgets checkboxes and...

Accessibility: Widgets: Further improve spacing between Widgets checkboxes and radio buttons in the admin interface.

Follow-up to [47598]:
- further improves the spacing after [47598] by better scoping the CSS to avoid layout glitches for custom widgets
- changes the RSS widget form to wrap the checkboxes in one single paragraph

Ideally, multiple related checkboxes and radio buttons should be grouped within a fieldset element with a legend. This will be addressed in a new Trac ticket.

Props mukesh27, SergeyBiryukov, sabernhardt.
Fixes #49228.

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git-svn-id: https://core.svn.wordpress.org/trunk@48158 1a063a9b-81f0-0310-95a4-ce76da25c4cd