Demo browser: re-add user stylesheets

Implemented in terms of user scripts.
Considering loading from a file URL would probably require
setting a different cross origin policy for the isolated
world we run this in, something we don't quite have yet, it
seems easier to just use an inline <style> block.

Change-Id: Ia6f7fbb96b925bcc3202b510689524abd93643fc
Reviewed-by: Andras Becsi <andras.becsi@theqtcompany.com>