Themability: Drag-and-drop CSS

The CSS selector on drag-and-drop handle needs to be a bit more specific.

Currently, we have just a.tabledrag-handle { .... }, which leaves absolutely no space for themes to define more different styles for links. If the theme have anything more specific than a bare a, it overrides the drag-and-drop definition as well (because theme's CSS come last); then the theme must deal with tabledrag things on its own, which is highly annoying because js-generated code may not be easily observed in a standard browser. Also reuqiring themes to theme standard features independently just to avoid changing them doesn't sound right to me.

My particular use case is as follows: I have underlined links in contents (pretty standard, I believe), but ONLY in contents. The site navigation (and any other links generated outside actual contents) is defined in a different way. To accomplish that, my style.css have simply: .content a { .... }, which I believe to be simple and clean solution. But however, it overrides the drag-and-drop definition now (see screenshot), requiring me to theme d-n-d handle explicitly in my theme, although I have absolutely no intention to change it in any way. Not nice.

The patch makes the selector in system.css one-step-more specific, allowing themes to have different a styles without interference.