Add support binding on-click handlers to other buttons than LEFT

One can now bind the left/middle/right mouse buttons to on-click. In
fact, you can have all three buttons bound to different handlers for
the same particle. The new syntax is

    on-click:
        left: <command>
        middle: <command>
        right: <command>

Leaving one out is the same thing as not mapping it at
all. Furthermore,

    on-click: <command>

is still valid, and is a shorthand for

    on-click:
        left: <commsnd>

tag.h

@@ -50,3 +50,6 @@ void tag_set_destroy(struct tag_set *set);
 
 /* Utility functions */
 char *tags_expand_template(const char *template, const struct tag_set *tags);
+void tags_expand_templates(
+    char *expanded[], const char *template[], size_t nmemb,
+    const struct tag_set *tags);