widget: Don't assume opacity is 100%

... just because there is no style context instantiated yet. Instead,
instantiate a style context during realize() and ask it.

Fixes problems with dim labels not being dimmed on first show.

Testcase included.

https://bugzilla.gnome.org/show_bug.cgi?id=735240

testsuite/reftests/Makefile.am

@@ -314,6 +314,9 @@ testdata = \
 	opacity.css \
 	opacity.ui \
 	opacity.ref.ui \
+	opacity-initial.css \
+	opacity-initial.ref.ui \
+	opacity-initial.ui \
 	overlay-no-main-widget.ref.ui \
 	overlay-no-main-widget.ui \
 	paned-undersized.css \