hasiXMLData() / hasBEXTData() were implemented as !data.isEmpty()
checks, which conflated in-memory payload with on-disk block presence.
That caused two wrong answers:
* setiXMLData("foo") on a file with no iXML block made hasiXMLData()
return true immediately, before save().
* A FLAC file carrying an iXML APPLICATION block with empty payload
round-tripped fine, but hasiXMLData() reported false.
Switch to the same model RIFF::WAV::File already uses: explicit
hasiXML / hasBEXT bool flags on FilePrivate, set during scan() when
the APPLICATION block is recognised, updated during save() after the
block is (re)written or omitted, and returned verbatim by the
accessors. New regression test pins down the before/after-save and
empty-block cases.
Refs: https://github.com/taglib/taglib/issues/1362
c32f7c7f86