Document which pieces of glib we're still using.

Looks like we can copy in a hash-table implementation, (from cairo,
say), and then a few _ascii_ functions from glib, (we'll need to
switch a few current uses if things like isspace, etc. to locale-
independent versions as well). So not too hard to free ourselves
of glib for now, (until we add GMime back in later, of course).

database.cc

@@ -24,7 +24,7 @@
 
 #include <xapian.h>
 
-#include <glib.h>
+#include <glib.h> /* g_strdup_printf, g_free, GHashTable */
 
 using namespace std;
 

message.c

@@ -22,7 +22,7 @@
 
 #include "notmuch-private.h"
 
-#include <glib.h>
+#include <glib.h> /* GHashTable */
 
 struct _notmuch_message {
     /* File objects */

notmuch.c

@@ -30,7 +30,7 @@
 #include <dirent.h>
 #include <errno.h>
 
-#include <glib.h>
+#include <glib.h> /* GIOChannel */
 
 #define ARRAY_SIZE(arr) (sizeof (arr) / sizeof (arr[0]))