recognize dock windows (and support matching them)

include/match.h

@@ -1,6 +1,14 @@
 #ifndef _MATCH_H
 #define _MATCH_H
 
+/*
+ * Initializes the Match data structure. This function is necessary because the
+ * members representing boolean values (like dock) need to be initialized with
+ * -1 instead of 0.
+ *
+ */
+void match_init(Match *match);
+
 /**
  * Check if a match is empty. This is necessary while parsing commands to see
  * whether the user specified a match at all.