PRIMARY selection handling for Wayland
                       Lyude <thatslyude@gmail.com> 2015
                              Sponsered by Red Hat
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                                --- Synopsis ---
  In order for the transition from X to Wayland to be easy for users, we have to
account for some of the smaller features of X that people will forever cling to.
One of these features is middle click pasting, where one application can have a
body of text selected, and that text will be pasted to another application by
clicking the middle mouse button as long as the text remains selected. This is a
port of that functionality for Wayland.

  The protocol adds a single global object called primary_selection_manager.
This singleton object provides notifications for when there's data available in
the primary buffer, when said data changes, and mechanisms for setting the
contents of the PRIMARY buffer.

                              --- The Protocol ---
primary_selection_manager:

  Handles notifying clients when they're allowed to read the contents of the
  primary buffer, along with handling setting the contents of the primary
  buffer.

  Requests:
    set_contents:
      Sets the current contents of the primary selection buffer. This request
      should only be valid when the client's surface currently has focus.
    Arguments:
      data:
        The wl_data_source containing the data to hold in the primary selection.

  Events:
    contents_changed:
      Means another client has set the contents of the primary selection. This
      means whatever we've set in the primary buffer has been replaced.

    contents_available:
      Sent when the client has permission to read from the primary selection if
      it chooses. This event is sent before any other events that may have
      triggered it. For example; if a middle click triggers primary selection
      pasting this event would be sent first, followed by the middle click
      event.
    Arguments:
      contents:
        A wl_data_offer containing the contents of the primary selection