Initial support for clipboard on OS X.

Rather than trying to get my existing hugely complicated X-style
clipboard code to somehow work with the Quartz GTK back end, I've
written an entirely new and much simpler alternative clipboard handler
usnig the higher-leve GtkClipboard interface. It assumes all clipboard
text can be converted to and from UTF-8 sensibly (which isn't a good
assumption on all front ends, but on OS X I think it's reasonable),
and it talks to GDK_SELECTION_CLIPBOARD rather than PRIMARY, which is
the only clipboard OS X has.

I had to do a fiddly thing to cope with the fact that each call to
gtk_clipboard_set_with_data caused a call to the clipboard clear
function left over from the previous set of data, so I had to avoid
mistaking that for a clipboard-clear for the _new_ data and
immediately deselecting it. I did that by allocating a distinct
placeholder object in memory for each instance of the copy operation,
so that I can tell whether a clipboard-clear is for the current copy
or a previous one.

This is only very basic support which demonstrates successful copying
and pasting is at least possible. For a sensible OS X implementation
we'll need a more believable means of generating a paste UI action
(it's quite easy to find a Mac on which neither Shift-Ins nor the
third mouse button even exists!). Also, after the trouble I had with
the clipboard-clear event, it's a bit annoying to find that it
_doesn't_ seem to get called when another application becomes the
clipboard owner. That may just be something we have to put up with, if
I can't find any reason why it's failing.

unix/unix.h

@@ -27,6 +27,8 @@
 #define OSX_META_KEY_CONFIG /* two possible Meta keys to choose from */
 /* this potential one of the Meta keys needs manual handling */
 #define META_MANUAL_MASK (GDK_MOD1_MASK)
+#define JUST_USE_GTK_CLIPBOARD_UTF8 /* low-level gdk_selection_* fails */
+#define DEFAULT_CLIPBOARD GDK_SELECTION_CLIPBOARD /* OS X has no PRIMARY */
 #endif
 
 struct Filename {