get_media_item(): add filter to better control "buttons" in media upload and make more performant if buttons are already set

[mitchoyoshitaka]

Two issues in get_media_item():

1. There's no good, clean way to modify the "buttons" (things which get stored lumped together in $form_fieldsbuttons?) in the Media Upload, except for the "delete" and "send" (aka "insert into post") which can be controlled via the 'get_media_item_args'. Add a filter for that.
2. Right now we construct the HTML snippets for different buttons, as $send, $delete, and $thumbnail... and then checking whether $form_fieldsbuttons? is already set or not, and then only using them if it's not already set. If a plugin already pre-specifies the buttons HTML, this computation is completely wasteful. Move this conditional earlier.
3. (Terribly minor:) If any of these buttons aren't actually used, <td class='savesend'/> is produced with extra whitespace in it. This is a reflection of the poor design of the code which prints this HTML.

Patch no longer applies cleanly (currently r20610).