Add uncompute functionality (WIP)
As discussed in Taipei we plan to do animations in Stylo on the Rust side. For cascading properly, we need to "uncompute" these,
i.e. convert them into a cascadeable specified value which when computed gets us the same thing again.
This patch starts work on this. Before writing uncompute code for everything, I'd like to check that this is an acceptable amount of mako magic,
and that the general design is okay (to avoid having to rewrite everything once it's done).
Preliminary r? @SimonSapin @birtles
<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/13228)
<!-- Reviewable:end -->
While we're here we also:
* remove any code conditional on style_struct.gecko_ffi_name, since all
style structs now do have a corresponding Geckos struct
* add new UIReset and XUL style structs, so that all Gecko structs are
now present (apart from Variables, which is special)
Sorry for the bulk size of this; I know already it's not going to be a fun thing to review. Nevertheless, it should be done so we finalize the split of this huge file into smaller, more maintable parts.
The content of stuff being moved to separate files is unchanged. Only some minor formatting changes have been made and similar, but nothing of particular interest. IMHO it should be safe to merge if all the tests are fine.
