This is needed primarily for a workaround for #7971. When importing a template
that has been exported on Ardour5 on MacOS we need to fix the paths of the
archive entries.
Later we can use this functionality also to handle imported templates if
templates with the same name already exist.
This commit only adds methods and members to FileArchive, it does not modify
anything to make regressions unlikely. This, however, leads to some duplicated
code. Eventually we should consolidate this a bit.
