Move some methods from RawLayoutElementHelpers to LayoutElementHelpers.
This is part of a long-term plan to ensure layout never has access to
unwrapped pointers to DOM objects. The remaining methods on the
RawLayoutElementHelpers trait are harder to move, because of the lifetimes in
their signatures.
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Don't generate useless InheritTypes interfaces
Interfaces with no descendant need neither a Base trait nor upcast functions, and interfaces with no ancestors neither a Derived trait nor downcast functions.
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This is part of a long-term plan to ensure layout never has access to
unwrapped pointers to DOM objects. The remaining methods on the
RawLayoutElementHelpers trait are harder to move, because of the lifetimes in
their signatures.
HTMLTableSectionElement improvements
'thead' and 'tfoot' now create instances of HTMLTableSectionElement
HTMLTableSectionElement.rows has been implemented
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Interfaces with no descendant need neither a Base trait nor upcast functions,
and interfaces with no ancestors neither a Derived trait nor downcast functions.
Implement <meta name=viewport> handling
Translate <meta name=viewport> as according to [CSS Device Adaption § 9](http://dev.w3.org/csswg/css-device-adapt/#viewport-meta)
Note: as the PR currently stands, handling `<meta name=viewport>` elements always occurs. This is probably not desired for some contexts (e.g. desktop), but I'm unsure of how to conditionally handle elements based on that.
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Refactor FormControl trait
The trait is now implemented for HTMLFooElement instead of
&HTMLFooElement and does no longer require an impl body.
Suggested by @Manishearth
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Simplify stacking context creation
Have Fragment::create_stacking_context understand which stacking
contexts need layers and which do not. This simplifies the way it is
called and eliminates a bunch of code.
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Prior to this commit, `str_join` would skip empty items at the start of
the `Iterator` until it found a non-empty item. This contradicts
`SliceConcatExt::join` which doesn't skip anything.
Brought up in:
https://github.com/servo/servo/pull/7776#issuecomment-144317281
Have Fragment::create_stacking_context understand which stacking
contexts need layers and which do not. This simplifies the way it is
called and eliminates a bunch of code.
