Accessibility: Widgets: Prevent checkboxes in widget control forms from...

Accessibility: Widgets: Prevent checkboxes in widget control forms from overlapping on smaller screens.

Props jaz_on, milindmore22, passoniate, audrasjb, carriganvb, afercia, SergeyBiryukov.
Fixes #49228.
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git-svn-id: https://core.svn.wordpress.org/trunk@47373 1a063a9b-81f0-0310-95a4-ce76da25c4cd