Update to gtk-rs 0.14, support mod-tap in backend code, and some code improvements

[ids1024]

Update from patched git build of gtk-rs to release 0.14, now that it is released an Ubuntu is packaging Rust 1.51.

This also includes the backend changes from #79, so that the Configurator can load a .json layout containing MT(...) bindings, and export/display like that if the keyboard is programmed with it.

This should have no visible impact on behavior, expect when a mod-tap binding is set from outside the GUI.

[leviport]

Mod-tap keys that I bind with QMK directly do show up, but I imagine we might want to find a different way to represent them when we make them an exposed option. I'm okay with this for now, though.

Importing and exporting .json layouts with MT's bound seems to be working nicely.

I did regression tests with Launch, oryp6, and a Nelson arduino, and everything appears to be working as expected.

[ids1024]

but I imagine we might want to find a different way to represent them when we make them an exposed option

Indeed. This representation seems good for the json format, once properly supported we'll probably want to show it differently.

(And if anyone has ideas for a good way to display it in the UI, but all means comment the suggestion on #79. Because I'm not really sure.)