dash: Ensure style for icon size computation

StIcon will skip loading the texture when its theme node is unset (which
may happen on style changes while the widget is hidden). While our size
request to compute the dash icon size will create the icon's theme node
if necessary (and of all its parents), a missing texture can still throw
off our computation.
Make sure this doesn't happen by ensuring the icon's style first, so the
texture is updated in response to StWidget::style-changed if necessary.

https://bugzilla.gnome.org/show_bug.cgi?id=745649