Menu: Add icon property

[redstrate]

This adds an icon that is displayed to the left of the title, and is also shown for Menus are inside of a parent menu.

Closes #7791

[ogoffart]

Thanks for this much appreciated patch.
Looks good to me, but I'm not the right person to judge the aesthetics, but I think it is ok for now.
Could you make sure that the gallery has at least one menu entry with one icon? (Also a disabled entry would be nice)

Looks like the CI is failing in the C++ tests.

   Error: /Users/runner/work/slint/cppbuild/api/cpp/generated_include/slint_builtin_structs_internal.h:139:5: error: unknown type name 'Image'
      Image icon;
      ^

This is because the builtin struct now contain an image. This header is generated in

slint/api/cpp/cbindgen.rs

Line 119 in ec1e125

writeln!(structs_priv, "// This file is auto-generated from {}", file!())?;

, and it should be sufficient to add an #include <slint_image.h> there.

Also the muda implementation in muda.rs is missing.

[ogoffart]

Thanks. Looks good to me.

The CI seems to expose more problem with missing include in some of our headers