Define use_glib in features.prf

When we use GStreamer we need to define USE(GLIB), we currently do this
separately in WTF and WebCore. This should instead be done centrally to
make the it consistent.

Change-Id: I079dbeaf99b091ea2ecd093bfef1428d963f8300
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>
Missing QWebSettings

We have enabled the Canvas Path Object as compile-time feature, but
being disabled by default we give the user no way of using it.

CSS Grid layout is also disabled by default, but is already exposed but
not documented.

Change-Id: Ie7a7ac4c2ad24978ed83996ac37327acb2870fe0
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>