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.c

@@ -533,3 +533,11 @@ tags_expand_template(const char *template, const struct tag_set *tags)
 
     return formatted.s;
 }
+
+void
+tags_expand_templates(char *expanded[], const char *template[], size_t nmemb,
+                      const struct tag_set *tags)
+{
+    for (size_t i = 0; i < nmemb; i++)
+        expanded[i] = tags_expand_template(template[i], tags);
+}