Add "New cross-axis mapping group" dialog to construct a group of mappings

For context, @m4rc1e is developing for Google Fonts a modest SPA web-app to allow type designers to category-tag design spaces with emotional classifications; eg if you search fonts.google.com today for [Calligraphy > Informal](https://fonts.google.com/?categoryFilters=Calligraphy:%2FScript%2FInformal) type families, then [Recursive](https://fonts.google.com/?categoryFilters=Calligraphy:%2FScript%2FInformal) won't show up in its region of design space to which that classification tag ought to apply, but that ought to work, so GF needs tagging of design sub spaces to make that work, and therefore needs a small app to author such tag data.

Following on from that and #2596 then I am reaching for a dialog similar to one that @m4rc1e has made to construct sets of mappings that cover all the univars, duovars, triovars, etc required to construct a non-hidden user axis from a set of hidden axes.

As an example, I added a 2 word spacing axes to Ammonia Delta (https://github.com/googlefonts/ammonia-delta/commit/0496de8e5e24077f661832cf6c909a7b81dcaf90), one parametric and one for users, and I want to blend the parametric one into my opsz, wght, wdth axes, and the user wordspace axis needs to have corners with all the user axes; and doing that is more tedious than it ought to be :)

I'll follow up with a screenshot/screencast of Marc's dialog for constructing a cross product of axis locations.

Start by reading issue #2596 and reviewing the existing dialog developed by @m4rc1e; the issue does not name a file, test, or entry point. Use the promised screenshot or screencast to clarify the interaction, then verify that the dialog can construct the required cross-axis mapping groups and cover the described axis combinations.

Written by the indexing model from the issue text.