Rename "val_feature_is_xlink" to "val_is_xlink" or "val_is_reference"

[yaozhihang]

Is val_is_xlink more generic and can also be used for e.g. shared address? maybe val_is_reference is better because the "xlink" looks still XML/GML encoding specific.

[clausnagel]

val_is_reference sounds good to me.