This commit changes mouse-based selection and middle click to use the
PrimarySelection.
CTRL-SHIFT-{C,V} use Clipboard.
{SHIFT,CTRL}-Insert use PrimarySelection.
`CompleteSelection` and `CompleteSelectionOrOpenLinkAtMouseCursor` now
require a parameter to specify the destination clipboard.
Removed the `default_clipboard_XXX` options added in
8dad34fa61 in favor of just explicitly
assigning the key/mouse bindings.
closes: #417