Twenty Fourteen: fixes for accent color functionality, props celloexpressions. See #25220.

 * Apply the primary accent color to the current-time indicator for the `mediaelements` players.
 * Fix instances of the custom accent color CSS overriding white or black link colors that should have no text-color change on hover.
 * Remove text color from main navigation hover states for submenus and from main navigation top-level items.
 * Change the background color a bit more on hover for main navigation submenus.

git-svn-id: https://develop.svn.wordpress.org/trunk@25612 602fd350-edb4-49c9-b593-d223f7449a82