Add 'backup-rust' and 'restore-rust' build targets

backup-rust moves src/rust up, out of the build directory and restore-rust
moves it back. For use by build automation.
Merge pull request #418 from brson/wip

Add 'backup-rust' and 'restore-rust' build targets
submodule: update rust-opengles
Add codegen support for dictionaries, and implement Event bindings as a side-effect. Add a stub EventTarget to allow the build to complete.
Remove unused mutable warning spam from generated code.
Merge pull request #422 from jdm/event

Add Event bindings and support for WebIDL dictionaries
layout: Refactor the RenderBox type to save memory and improve the coding style of box.rs
Merge pull request #419 from pcwalton/render-box-refactor

layout: Refactor the RenderBox type to save memory and improve the coding style of box.rs