Replace usages of SplitBoxResult with option types
Flatten some pattern matching
Add ref test for nested inline elements
Avoid splitting boxes if possible
Add doc comments explaining some return types
Pass external dependencies to submodule Makefiles

Part of the fix for #2137.
Add some fmt::show implementations to make debugging layout easier.
Initial spec-incompliant implementation of default click action for anchor elements.

This is cherry-picked from https://github.com/mozilla/servo/pull/1688:

  * Initial spec-incompliant implementation of default click action for anchor elements.
  * Add documentation; gut the new document URL loading method
    and move it all into the new Window method.
  * Add test for default event prevention.

Original developer: Josh Matthews <josh@joshmatthews.net>
Add LIBRARY_PATH env var to help the linux build slave.
auto merge of #2479 : metajack/servo/linux-library-path, r=metajack
Replace deprecated rev_iter() by iter().rev().
auto merge of #2441 : saneyuki/servo/event, r=jdm

- Fix #2118
- Take over from #2293
Implement CustomEvent (#2173)
auto merge of #2374 : Manishearth/servo/customevent, r=jdm

This was previously PR'd on #2218
auto merge of #2468 : glennw/servo/show-traits, r=jdm
auto merge of #2471 : bjz/servo/split-to-width, r=pcwalton

@pcwalton r?
Remove the workaround for Rust issue 10683.
auto merge of #2472 : mbrubeck/servo/2137-done-deps, r=metajack

Part of the fix for #2137.  This allows submodules to correctly recompile (or not) based on all of the dependency information from `mk/sub.mk`.  Along with this change, we will also need to make changes to each of the affected submodules' make files, for example in `rust-azure/Makefile.in`:

```diff
-libazure.dummy: azure.rc $(RUST_SRC) libazure.a ../../skia/skia/libskia.a
+libazure.dummy: azure.rc $(RUST_SRC) libazure.a $(EXT_DEPS)
```

r? @metajack
auto merge of #2482 : Ms2ger/servo/rev_iter, r=jdm
auto merge of #2483 : zwarich/servo/remove-10683-workaround, r=jdm