Refactor flow tree construction and actually use display property.
auto merge of #508 : eric93/servo/refactor_flowtree_construction, r=pcwalton

Eliminates a level of indirection in flow tree construction and uses 'display' to construct flows.
Implement basic last-resort font selection.
auto merge of #511 : caitp/servo/issue-479, r=jdm

Return lists of last-resort font family names in platform-specific font_list code, so that they can be appended to the end of FontGroup and used if the requested font faces are not available.
Merge remote-tracking branch 'origin/master' into pcwalton-master

Conflicts:
	src/components/main/layout/box_builder.rs