Stop using GtkDialog (for most purposes) in GTK 3!

They've now deprecated gtk_dialog_get_action_area, because they really
want a dialog box's action area to be filled with nothing but buttons
controlled by GTK which end the dialog with a response code. But we're
accustomed to putting all sorts of other things in our action area -
non-buttons, buttons that don't end the dialog, and sub-widgets that
do layout - and so I think it's no longer sensible to be trying to
coerce our use cases into GtkDialog.

Hence, I'm introducing a set of wrapper functions which equivocate
between a GtkDialog for GTK1 and GTK2, and a GtkWindow with a vbox in
it for GTK3, and I'll lay out the action area by hand.

(Not everything has sensible layout and margins in the new GTK3 system
yet, but I can sort that out later.)

Because the new functions are needed by gtkask.c, which doesn't link
against gtkdlg.c or include putty.h, I've put them in a new source
file and header file pair gtkmisc.[ch] which is common to gtkask and
the main GTK edifice.

unix/gtkmisc.h

@@ -0,0 +1,15 @@
+/*
+ * Miscellaneous helper functions for GTK.
+ */
+
+#ifndef PUTTY_GTKMISC_H
+#define PUTTY_GTKMISC_H
+
+GtkWidget *our_dialog_new(void);
+void our_dialog_add_to_content_area(GtkWindow *dlg, GtkWidget *w,
+                                    gboolean expand, gboolean fill,
+                                    guint padding);
+void our_dialog_set_action_area(GtkWindow *dlg, GtkWidget *w);
+GtkBox *our_dialog_make_action_hbox(GtkWindow *dlg);
+
+#endif /* PUTTY_GTKMISC_H */