Fixes for #137856 & #137507 were workarounds to ignore the
PROP_PATH_SUPPORTS_BLEND_RELATIVE for CollectionExport however they
were error prone - especially #137856 as it relied on the surrounding
buttons.
Instead of adding logic to ignore the flag for this specific case,
add a property to the CollectionExport which can have it's flags
and use this instead of the operators "filepath" property.
Internally this wraps the same IDProperty data used by the operator.
Ref !137883
e8f1473da6