modules: move 'content' and 'anchors' to MODULE_COMMON_ATTRS

All modules implement these. The only one that differs is the i3
module. It still implements them, but 'content' uses an internal
verifier function.

module.h

@@ -34,6 +34,8 @@ struct exposable *module_begin_expose(struct module *mod);
 
 /* List of attributes *all* modules implement */
 #define MODULE_COMMON_ATTRS                        \
+    {"content", true, &conf_verify_particle},      \
+    {"anchors", false, NULL},                      \
     {"font", false, &conf_verify_font},            \
     {"foreground", false, &conf_verify_color},     \
     {NULL, false, NULL}