Expose state override for HeaderResponse (#4200)

I'm trying to create some custom collapsing headers that add additional
buttons inside the header itself. Because of this, I load the
`CollapsingState` in my special widget manually. But because
`HeaderResponse` owns the `Ui` and the `CollapsingState`, there is no
way for me to open/close the collapsing header based on response of the
inner widget.

Initially, I considered just exposing `state` member of
`HeaderResponse`, but that exposes too much of the API at the wrong
time, in my opinion.

So instead I found it'd be safer to just expose the open/close API to
the response itself, and that's what this PR does.

crates/egui/src/containers/collapsing_header.rs

@@ -272,6 +272,18 @@ pub struct HeaderResponse<'ui, HeaderRet> {
 }
 
 impl<'ui, HeaderRet> HeaderResponse<'ui, HeaderRet> {
+    pub fn is_open(&self) -> bool {
+        self.state.is_open()
+    }
+
+    pub fn set_open(&mut self, open: bool) {
+        self.state.set_open(open);
+    }
+
+    pub fn toggle(&mut self) {
+        self.state.toggle(self.ui);
+    }
+
     /// Returns the response of the collapsing button, the custom header, and the custom body.
     pub fn body<BodyRet>(
         mut self,