pluma-view: use static variables for cssprovider and settings

We need to create these once and keep them as long as Pluma is running.
Also do not use newly deprecated(in GTk 3.23) gtk_css_provider_get_default()

pluma/pluma-view.c

@@ -686,7 +686,7 @@ pluma_override_font (const gchar          *item,
 		     PangoFontDescription *font)
 {
 	static gboolean provider_added = FALSE;
-	GtkCssProvider *provider;
+	static GtkCssProvider *provider; /*We need to keep this as long as Pluma is running*/
 	gchar          *prov_str;
 	gchar          *css;
 	gchar          *family;
@@ -709,7 +709,8 @@ pluma_override_font (const gchar          *item,
 				pango_font_description_get_size (font) / PANGO_SCALE,
 				pango_font_description_get_size_is_absolute (font) ? "px" : "pt");
 
-	provider = gtk_css_provider_get_default ();
+	if (!provider_added)
+		provider = gtk_css_provider_new ();
 
 	prov_str = gtk_css_provider_to_string (provider);
 
@@ -729,7 +730,7 @@ pluma_override_font (const gchar          *item,
 
 	if (!provider_added)
 	{
-		GSettings *settings;
+		static GSettings *settings; /*We need this for the life of the provider and program*/
 		settings = g_settings_new ("org.mate.interface");
 		g_signal_connect (settings,
 				  "changed::" "font-name",