For hyperlinks, add context menu item "Copy link location"

[Lewis Rosenthal]

Now that we have hyperlinks working and displaying in tooltips (see #356), as well as handy context menus, we should make the context menu more useful by offering to copy the URI to the clipboard.

Currently, selecting Copy from the context menu copies some of the underlying text in the document, which by itself is sub-optimal.

I'm only marking this as an RFE for the PDF plugin, though the implementation may be useful for other document types, as well (DjVu, perhaps).