We use KFileItemListProperties::supportsWriting() to check whether a folder
is read-write and can be used as extract destination, but this function returns
false if the archive is read-only. A read-only archive can still be extracted,
so we need to implement our own check for the writability of parent folders.

Check that https://bugs.kde.org/show_bug.cgi?id=370948 is fixed.
Check that the action is still disabled in read-only folders.