contextual_help filter not very useful

[filosofo]

The new (as of 2.7) 'contextual_help' filter just allows you to make boolean changes, viz. whether or not to show the contextual_help. This makes it almost useless.

Instead, it should allow plugins to filter the help content, and it should pass useful information. (The $screen variable it uses is not very specific).

That way plugins can add help information to specific pages.

My patch passes the "suffix" of the page to the contextual_help filter, so plugins can easily add help to specific pages.

Also, as a bonus I cleaned up some unnecessarily verbose related code:

• wp-admin/admin-header.php was making the suffixes into an array consisting of the suffix and a blank string, in what I guess was an attempt to save code but actually generated more lines.
• the logic adding tinymce to the correct admin_head action was overkill.

[ryan]

(In [9636]) screen_meta() and contextual help improvements from filosofo. fixes #8172