Sort dock clients by class and instance

This is similar to #3820 but does not use qsort but an insertion sort in
con_attach.

Since each bar block automatically gets its own incremental bar id,
bards end up being sorted according to their definition order in the
config file.

For i3bar, the WM_CLASS is modified to include an instance name which
depends on the bar_id. This could be useful for other reason, e.g. users
targeting a specific bar instance.

Fixes #3491

libi3/safewrappers.c

@@ -110,3 +110,20 @@ ssize_t swrite(int fd, const void *buf, size_t count) {
     else
         return n;
 }
+
+/*
+ * Like strcasecmp but considers the case where either string is NULL.
+ *
+ */
+int strcasecmp_nullable(const char *a, const char *b) {
+    if (a == b) {
+        return 0;
+    }
+    if (a == NULL) {
+        return -1;
+    }
+    if (b == NULL) {
+        return 1;
+    }
+    return strcasecmp(a, b);
+}