Editor scrolling: Remember and restore scroll position for visual and text modes.

This prevents the editor from jumping off screen when the rendered visual content is significantly taller than plain text, e.g. when there are views present.

This is also a close restoration of 3.9 behavior, some of which was browser-specific.

props avryl.
fixes #29338.

Built from https://develop.svn.wordpress.org/trunk@29693


git-svn-id: http://core.svn.wordpress.org/trunk@29468 1a063a9b-81f0-0310-95a4-ce76da25c4cd