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

include/libi3.h

@@ -181,6 +181,12 @@ ssize_t writeall_nonblock(int fd, const void *buf, size_t count);
  */
 ssize_t swrite(int fd, const void *buf, size_t count);
 
+/**
+ * Like strcasecmp but considers the case where either string is NULL.
+ *
+ */
+int strcasecmp_nullable(const char *a, const char *b);
+
 /**
  * Build an i3String from an UTF-8 encoded string.
  * Returns the newly-allocated i3String.