mu: Default to XDG Base Directory Specification

Instead of using ~/.mu, use the XDG Base Directory Specification, typically:
  ~/.cache/xapian
  ~/.cache/mu.log
  ~/.cache/parts
  ~/.config/bookmarks

Update dependencies, documentation.

lib/mu-util.c

@@ -194,32 +194,6 @@ mu_util_guess_maildir (void)
 	return NULL;
 }
 
-
-gchar*
-mu_util_guess_mu_homedir (void)
-{
-	const char* home;
-	const gchar *hdir1;
-
-	/* first, try MU_HOME */
-	hdir1 = g_getenv ("MU_HOME");
-
-	if (hdir1 && mu_util_check_dir (hdir1, TRUE, FALSE))
-		return g_strdup (hdir1);
-
-	/* then, g_get_home_dir use /etc/passwd, not $HOME; this is
-	 * better, as HOME may be wrong when using 'sudo' etc.*/
-	home = g_get_home_dir ();
-
-	if (!home) {
-		MU_WRITE_LOG ("failed to determine homedir");
-		return NULL;
-	}
-
-	return g_strdup_printf ("%s%c%s", home ? home : ".",
-				G_DIR_SEPARATOR, ".mu");
-}
-
 gboolean
 mu_util_create_dir_maybe (const gchar *path, mode_t mode, gboolean nowarn)
 {