ComboBox is now wrapped in `horizontal_wrap` container

**Describe the bug**

When a combo boxes are placed in wrapping container, they do not wrap when the container size shrinks.

**To Reproduce**
I modified Layout Test from demo app in the following way (just added few buttons and combo boxes):

```diff
diff --git a/crates/egui_demo_lib/src/demo/layout_test.rs b/crates/egui_demo_lib/src/demo/layout_test.rs
index a5726c7a..ee04f0a1 100644
--- a/crates/egui_demo_lib/src/demo/layout_test.rs
+++ b/crates/egui_demo_lib/src/demo/layout_test.rs
@@ -180,5 +180,42 @@ fn demo_ui(ui: &mut Ui) {
let mut dummy = false;
ui.checkbox(&mut dummy, "checkbox");
ui.radio_value(&mut dummy, false, "radio");
- let _ = ui.button("button");
+ let _ = ui.button("button1");
+ let _ = ui.button("button2");
+ let _ = ui.button("button2");
+ let _ = ui.button("button2");
+ let _ = ui.button("button2");
+ let _ = ui.button("button2");
+ let _ = ui.button("button2");
+ let _ = ui.button("button2");
+ let _ = ui.button("button3");
+
+ let alternatives = ["a", "b", "c", "d"];
+ let mut selected = 2;
+ egui::ComboBox::from_label("combobox1").show_index(
+ ui,
+ &mut selected,
+ alternatives.len(),
+ |i| alternatives[i]
+ );
+
+ let mut selected = 2;
+ egui::ComboBox::from_label("combobox2").show_index(
+ ui,
+ &mut selected,
+ alternatives.len(),
+ |i| alternatives[i]
+ );
+
+ let mut selected = 2;
+ egui::ComboBox::from_label("combobox3").show_index(
+ ui,
+ &mut selected,
+ alternatives.len(),
+ |i| alternatives[i]
+ );
+ let _ = ui.button("button2");
+ let _ = ui.button("button2");
+ let _ = ui.button("button2");
+ let _ = ui.button("button3");
}
```

**Expected behavior**
Checkboxes should behave like other widgets.

**Screenshots**
I recorded the modified demo app: https://youtu.be/O5CnZ1KF8D4

Start with crates/egui_demo_lib/src/demo/layout_test.rs and run the layout demo using the reproduction described in the issue. Compare ComboBox behavior with buttons and checkboxes inside a shrinking wrapping container. Done means ComboBox widgets wrap like the other widgets when the container becomes narrower.

Written by the indexing model from the issue text.