Add a new capability VD_AGENT_CAP_CLIPBOARD_SELECTION.

When both client and servers have this capability, the
VDAgentClipboard* messages MUST be prepended with a uint8_t indicating
which clipboard selection to operate + 3 bytes stuffing for alignment
that could be used for future capabilities or extensions.

A few clipboard selection are defined according to X11/Gtk scheme:

- VD_AGENT_CLIPBOARD_SELECTION_CLIPBOARD: the default clipboard,
  implemented by most OS to deal with explicit Copy/Paste operations.

- VD_AGENT_CLIPBOARD_SELECTION_PRIMARY: the PRIMARY clipboard, used
  for mouse selections.

- VD_AGENT_CLIPBOARD_SELECTION_SECONDARY: the SECONDARY clipboard.

See also:
http://www.freedesktop.org/wiki/Specifications/ClipboardsWiki