#389 [preliminary cleanup] Standardise metadat for custom field use

In digging into #389 I found a major complexity in fixing it was inconsistent metadata available at different
points in the code. This simply adds metadata to the 'inner-most place' & makes the BAO_CustomQuery object use that

I didn't remove the options loop because I wanted to leave that out of scope as I can't see how / if it is used

[test] add test for getImportableFields