Make ErrorEvent::new take cancelable and bubbling enums
auto merge of #4372 : thiagopnts/servo/errorevent, r=Ms2ger

Fixes #4174.
style: Move presentational hint synthesis into `legacy.rs`.
style: Parse the legacy `border` attribute per the legacy HTML specification.

Additionally, this patch cleans up some miscellaneous formatting issues
and refactors files in `layout/css/` somewhat to eliminate needless
levels of indirection. It also fixes our handling of presentational
hints that only apply if border is nonzero.
style: Parse the legacy `bgcolor` attribute per the HTML5 specification.

Additionally, this patch cleans up some miscellaneous formatting issues.
style: Implement basic column spans.

This patch provides some of the groundwork for column spans greater than
1. It implements the column-span CSS property (prefixed so as not to be
exposed to content) as well as the corresponding colspan attribute;
although the former is not well-specified outside of CSS multi-column
layout, INTRINSIC refers to it. Although width is distributed to
spanning columns, they do not yet contribute minimum and preferred
widths; this will be implemented in a follow-up.

Additionally, this patch cleans up some miscellaneous formatting issues
and improves the handling of table rowgroups.
style: Implement quirks mode rules.
Address review comments related to `column-span`
Address review comment related to where "border" goes
Remove stray commented out code
Address more review comments
Address more review comments
Introduce a "parser context" to selector parsing so we can avoid
exposing the "-servo" selector to content
Add a new reftest and address incorrect comment
Fix crash
Add missing file
Fix rebase fallout
Address review comments
Fix fixed layout bug