scale: Decompile non-translatable labels too

While working on the previous commit, I noticed Gtk.Scale specially
handles translator comments, but more importantly, that it does not put
non-translatable labels in its decompiled output.

This is wrong, as it's not unlikely that Gtk.Scale marks would use a
purely numeric string, which should not be lost upon decompilation.

Add a test for this case too.

tests/samples/scale_marks.blp

@@ -4,6 +4,7 @@ Scale {
   marks [
     mark (-1, bottom),
     mark (0, top, _("Hello, world!")),
+    mark (1, bottom, "1"),
     mark (2),
   ]
 }

tests/samples/scale_marks.ui

@@ -10,6 +10,7 @@ corresponding .blp file and regenerate this file with blueprint-compiler.
     <marks>
       <mark value="-1" position="bottom"></mark>
       <mark value="0" position="top" translatable="yes">Hello, world!</mark>
+      <mark value="1" position="bottom">1</mark>
       <mark value="2"></mark>
     </marks>
   </object>