qt,macos,config: lookup cat from subcat

Premise: 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.

This:

• Does a little minimal prep work in vlc_config_cat.h towards storing subcat->cat mappings (since MR !307 has stalled).
• Adds the lookup function and data.
• Converts the qt and macos interface modules to make use of it.

The purge of set_category() will follow separately.