This commit adds a need_index prop to the style system, and autogenerates
iterators, and a get_xxx_mod(i) function from a get_xxx_prop() and
get_xxx_at(index) functions.
This allows us to (finally!) take rid of the as_servo() hack. There are a few
unimplemented clones, but I'm just too lazy for that right now.
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.
