change the way font specifications are used in UI config files

If no font family is specified, enforce use of Sans to match GTK behaviour (which we inadvertently relied on, it appears)

gtk2_ardour/utils.cc

@@ -206,6 +206,18 @@ xpm2rgba (const char** xpm, uint32_t& w, uint32_t& h)
 	return (savergb);
 }
 
+Pango::FontDescription
+sanitized_font (std::string const& name)
+{
+	Pango::FontDescription fd (name);
+
+	if (fd.get_family().empty()) {
+		fd.set_family ("Sans");
+	}
+
+	return fd;
+}
+
 Pango::FontDescription
 get_font_for_style (string widgetname)
 {