Accept string literals for parseable types

GtkBuilder knows some class-specific parsing methods, so we can use
string literals for these types and pass them straight to the XML.

tests/samples/parseable.blp

@@ -0,0 +1,5 @@
+using Gtk 4.0;
+
+Gtk.Shortcut {
+  trigger: "Escape";
+}

tests/samples/parseable.ui

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<interface>
+  <requires lib="gtk" version="4.0"/>
+  <object class="GtkShortcut">
+    <property name="trigger">Escape</property>
+  </object>
+</interface>

tests/test_samples.py

@@ -104,6 +104,7 @@ class TestSamples(unittest.TestCase):
         self.assert_sample("layout")
         self.assert_sample("menu")
         self.assert_sample("object_prop")
+        self.assert_sample("parseable")
         self.assert_sample("property")
         self.assert_sample("signal")
         self.assert_sample("size_group")