Allow definition of external style sheet in multi-block html template

[James Yong]

Hi all,

Some external style sheets defined in layoutSettings.styleSheets are 
page-specific.
E.g. jquery.jgrowl-1.4.6.min.css, select2-4.0.6.css

Propose new function to set the external style sheet in multi-block html 
template.

Regards,
James