GafferUI : Ensure forward slashes in Style Sheet
GafferUI : Ensure forward slashes in Style Sheet

- Windows `GAFFER_ROOT` will use backslashes which makes style sheet URLs fail to resolve.

johnhaddon/gaffer,GafferHQ/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,andrewkaufman/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,GafferHQ/gaffer