Support libhandy hardcoded styles

This is needed to port libhandy classes.

gtk/Makefile.am

@@ -601,6 +601,7 @@ gtk_private_h_sources =		\
 	gtkwindowprivate.h	\
 	gtktreemenu.h		\
 	gdkpixbufutilsprivate.h \
+	hdy-style-private.h	\
 	open-type-layout.h	\
 	language-names.h	\
 	script-names.h
@@ -1325,6 +1326,10 @@ gtk.gresource.xml: Makefile.am inspector/Makefile.inc
 	  n=`basename $$f`; \
 	  echo "    <file preprocess='xml-stripblanks'>ui/$$n</file>" >> $@; \
 	done; \
+	for f in $(srcdir)/style/*.css; do \
+	  n=`basename $$f`; \
+	  echo "    <file>style/$$n</file>" >> $@; \
+	done; \
 	for s in 16x16 22x22 24x24 32x32 48x48; do \
 	  for c in actions status categories; do \
 	    for f in $(srcdir)/icons/$$s/$$c/*.png; do \

gtk/hdy-style-private.h

+/*
+ * Copyright (C) 2019 Purism SPC
+ *
+ * SPDX-License-Identifier: LGPL-2.1+
+ */
+
+#pragma once
+
+#include "gtkstyleprovider.h"
+
+G_BEGIN_DECLS
+
+/* The style provider priority to use for libhandy widgets custom styling. It is
+ * higher than settings but lower than applications, so application developers
+ * can nonetheless apply custom styling on top of it. */
+#define GTK_HDY_STYLE_PROVIDER_PRIORITY (GTK_STYLE_PROVIDER_PRIORITY_SETTINGS + 1)
+
+G_END_DECLS