wgui: parser: template injection support

wlx-overlay-s/src/assets/key.xml

@@ -1,7 +1,23 @@
 <layout>
+  <!-- Template of a single keyboard key. -->
   <template name="Key">
-    <rectangle color="#FF0000" width="32" height="32" margin="4">
-
+    <rectangle
+      border_color="#0044CC"
+      border="2"
+      round="8"
+      color="#000A1C"
+      color2="#000002"
+      gradient="vertical"
+      width="${width}"
+      height="${height}"
+      min_width="${width}"
+      min_height="${height}"
+      max_width="${width}"
+      max_height="${height}"
+      margin="4"
+      align_items="center"
+      justify_content="center">
+      <label color="#FFFFFF" text="${text}" size="24" />
     </rectangle>
   </template>
 </layout>