st/widget: Fix applying :insensitive to initially unreactive widgets

We are applying the :insensitive pseudo class to unreactive widgets, or at least we are supposed to. As we currently only update the style on notify, we don't apply it to initially unreactive widgets.

This was covered up partially until recently when Clutter started to use G_PARAM_EXPLICIT_NOTIFY. Before that, the notify handler would run when explicitly setting :reactive to FALSE at construction time.

Make sure we always apply the pseudo class correctly by updating it after construct properties have been set.

Fixes #3685 (closed)