Add monitor interface

Add an interface to monitor devices and helper program
Add v4l2 monitor
Add simple dictionary with key/value pairs

spa/include/spa/clock.h

@@ -57,6 +57,12 @@ struct _SpaClock {
   /* the total size of this clock. This can be used to expand this
    * structure in the future */
   size_t size;
+  /**
+   * SpaClock::info
+   *
+   * Extra information about the clock
+   */
+  const SpaDict *info;
   /**
    * SpaClock::state:
    *