Insert Media modal: IE 7, 8, and 9 issues

[SergeyBiryukov]

1. The vertical line on the left is displayed in IE 9, but not in IE 8 (see 22446.ie8.png​, 22446.ie9.png​). [22610]

2. "Drop files anywhere to upload" shouldn't be displayed in IE 7/8/9, as it doesn't support drag-and-drop (at least it doesn't on wp-admin/media-new.php). [22609]

3. IE 7 needs some attention for the gallery view, see 22446.ie7_gallery.png​

4. IE 7 doesn't display the selected images (22446.ie7_selected_images.png​) and in IE 9 you can see the second line (22446.ie9_selected_images.png​)

5. In IE 7 and IE 8 you can't see which image is currently selected (blue border is missing), see 22446.ie8_selected_image.png​

6. In IE 7 Embed from URL doesn't work. Nothing happens after inserting an URL.

7. When uploading an image in IE 7 a pop up comes up which wants to download admin-ajax.php. The file includes the JSON respond.

8. IE 8: Upload an image, switch to media library, try to change the filter dropdown. It will open the upload dialog.

The missing borders around the images are a result of the use of box-shadow, which IE 7 and 8 doesn't support. I think we should try to use real CSS borders here.

In 22609:

Media: Only display "Drop files" text when the browser supports drag and drop. see #22446, #21390.

In 22610:

Media: Add an explicit border to menus to show a divider in older browsers. see #22446, #21390.

In 22611:

Media: The zIndex arms race.

Adjusts zIndexes to be not-as-absurd as before, in the process moving the modal below the plupload flash shims (which do not have customizable zIndexes or classes, sadly).

fixes #22441, see #22446, #21390.

In 22612:

Media: zIndex peace was only temporary.

Revert [22611] in favor of using plupload's container setting. Turns out, plupload's flash shim is forced to the exact same index as the admin bar, so balancing the two is better solved by nesting the shim.

see #22441, #22446, #21390.