Support default-font-* properties in Live-Preview

[tronical]

... by changing the resolution for the WindowItem to traverse the item tree from the current item, instead of going to the window.

This doesn't quite fix #4298 because rem resolution is still missing. That requires the built-in default font size function to be fixed as well, which is non-trivial.

cc #4298

[ogoffart]

No tests?

Could you make a test that involve a PopupWindow. I have a suspicion that it may not work because the PopupWindow inject a WindowItem that doesn't define these default-font-* properties.

eg:

export component TestCase inherits Window {
   default-font-size: 140px;
   PopupWindow {
      // the font size of this text must be large
      Text { text: "hello"; } 
   }
}
``

[tronical]

No tests?

Could you make a test that involve a PopupWindow. I have a suspicion that it may not work because the PopupWindow inject a WindowItem that doesn't define these default-font-* properties.

eg:

export component TestCase inherits Window {
   default-font-size: 140px;
   PopupWindow {
      // the font size of this text must be large
      Text { text: "hello"; } 
   }
}
``

That's an excellent point - this is indeed also not handled (besides the rem issue).

[tronical]

Rebased to resolve conflicts and to implement the suggested change to to use &ItemRc in the vtable. Tests and PopupWindow fixes are still missing.

[tronical]

PopupWindow fixes are in, as discussed with run-time resolution. rem isn't working yet as we call sp::WindowInner::from_pub(#window_adapter_tokens.window()).window_item().unwrap().as_pin_ref().default_font_size().get() instead of starting at self_rc or so. I'll keep that out of this PR for now.