Issue #38, Issue #61 & Issue #70

Also a fix for the Unused CSS extension, preventing a
NullReferenceException when adding a LESS mixin
Project rel paths (/...) in ToAbsoluteFilePath
Relative url + preview fix for CSS file picker
Fixed embed smart tag, more unused CSS fixes
HtmlGoToDefinitionFix for render directives
Last of the ToAbsoluteFilePath usage updates
More pathing fixes