Change the layout of modal dialogs a little bit:
 - Remove the 2px inner padding around the entire dialog.
 - The header no longer has rounded borders on the bottom.
 - Make the dialog border darker.
 - Adjust dialog borders so the gap between the dialog border and the header is less visible. Apparently most browsers have bugs that let the background color/content of child elements bleed over rounded corners.

git-svn-id: https://plugins.svn.wordpress.org/admin-menu-editor/trunk@943344 b8457f37-d9ea-0310-8a92-e5e31aec5664