TinyMCE: revert style for the blocks drop-down in the toolbar, limit the...

TinyMCE: revert style for the blocks drop-down in the toolbar, limit the styles imported for the same drop-down, make the menu highlight color grey. Part props avryl, see #26952
Built from https://develop.svn.wordpress.org/trunk@27193


git-svn-id: https://core.svn.wordpress.org/trunk@27052 1a063a9b-81f0-0310-95a4-ce76da25c4cd