wgui: Separate user and wgui assets, topmost widgets (poc)

wgui/assets/wgui/close.svg

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+<svg xmlns="http://www.w3.org/2000/svg" width="32" height="32" viewBox="0 0 24 24">
+  <path fill="white" d="M8.4 17L7 15.6l3.6-3.6L7 8.425l1.4-1.4l3.6 3.6l3.575-3.6l1.4 1.4l-3.6 3.575l3.6 3.6l-1.4 1.4L12 13.4z" />
+</svg>

wgui/assets/wgui/window_frame.xml

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+<layout>
+  <elements>
+    <rectangle
+      flex_direction="column"
+      round="8"
+      border="2"
+      border_color="#778899"
+      color="#001122ee"
+      padding="2">
+
+      <!-- window title -->
+      <rectangle width="100%" height="100%" round="4" align_items="center" justify_content="space_between"
+        gradient="vertical" color="#224466" color2="#113355">
+        <label margin_left="8" text="Window title" weight="bold" />
+        <sprite src_internal="wgui/close.svg" width="24" height="24" />
+      </rectangle>
+
+      <!-- content itself -->
+      <div width="100%" height="100%" padding="8" gap="4" flex_direction="column">
+        <label text="Window content" />
+        <Button color="#9911AA" text="I'm clickable." width="128" height="24" />
+      </div>
+    </rectangle>
+  </elements>
+</layout>