config: lookup cat from subcat

having `set_category()` + `set_subcategory()` pairs of calls to setup the
location in the preferences tree where a plugin node needs to be attached
to, with corresponding pairs of option records in the plugin option sets,
is silly, inefficient and bug prone. all we need is the subcat records
and we can look up the corresponding cat from the data table when it
comes to time to build the tree.

here we take a step forward to address this, adding cat data to the subcat
data table, and a lookup function making use of it.

note, qt and macos tree construction are done a little differently from
each other, so while the change here is entirely sufficient to rid the core
option set of category hints as far as the macos code is concerned, more
work in a subsequent commit will be needed for qt. thus the purge of
`set_category()` must be postponed for a later commit and so for now we
must just ignore those records.

a subcat table entry was added for `SUBCAT_HIDDEN` since for now the qt
code could potentially look this up and we should preserve the existing
behaviour for it for now.