decompiler: Allow and ignore <placeholder/>

Some existing files contain <placeholder/> tags, which should be
permitted but ignored.

blueprintcompiler/decompiler.py

@@ -238,6 +238,11 @@ def decompile_requires(ctx, gir, lib=None, version=None):
     return gir
 
 
+@decompiler("placeholder")
+def decompile_placeholder(ctx, gir):
+    pass
+
+
 @decompiler("property", cdata=True)
 def decompile_property(ctx, gir, name, cdata, bind_source=None, bind_property=None, bind_flags=None, translatable="false", comments=None, context=None):
     name = name.replace("_", "-")

tests/samples/placeholder.ui

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<interface>
+  <requires lib="gtk" version="4.0"/>
+  <object class="GtkBox">
+    <child>
+      <placeholder/>
+    </child>
+  </object>
+</interface>

tests/samples/placeholder_dec.blp

@@ -0,0 +1,4 @@
+using Gtk 4.0;
+
+Box {
+}

tests/test_samples.py

@@ -207,6 +207,7 @@ class TestSamples(unittest.TestCase):
         self.assert_decompile("layout_dec")
         self.assert_decompile("menu_dec")
         self.assert_decompile("property")
+        self.assert_decompile("placeholder_dec")
         self.assert_decompile("signal")
         self.assert_decompile("strings")
         self.assert_decompile("style_dec")