To implement libcss's select handler API, we need to implement two
functions that get/set "userdata" on a node. The corresponding libdom
API allows you to set arbitrary data in the form of key->value pairs;
so to identify THIS particular data, we need to use the same string
each time. For that reason we intern "_libcss_user_data" and call it
the "userdata_key".
SVGTINY_STRING_ACTION2(zero_percent,0%)
SVGTINY_STRING_ACTION2(hundred_percent,100%)
SVGTINY_STRING_ACTION2(universal,*)
+SVGTINY_STRING_ACTION2(userdata_key,_libcss_user_data)
#undef SVGTINY_STRING_ACTION