Response.context_menu now returns the response of the context menu,…

… if open (emilk#3904)

Hi everyone! It's a great pleasure to work with such a library. It feels
like a breath of fresh air!

**Problem:**

The current implementation of `context_menu` consumes `self` and returns
it. However, the underlying `menu::context_menu` requires only a
reference to `self`, so it is safe to change the `context_menu`
signature.

**Fix:**

1. Change signature to take a ref to `self`. 
2. Return the `Option<InnerResponse<()>>` from the underlying method
call

**Pros:**

1. No `let response =  response.context_menu(|| {...})` pattern
2. Better consistency with other `show`-like methods, as it is in the
`Window::show`
3. Less ownership gymnastics

**Cons:**
1. Breaking change
2. Smth else what I don't see ?

**Additional info:**
- This method is also addressed in [this
PR](emilk#857).
- `check.sh` fails only on `cargo check --quiet -p eframe
--no-default-features --features wgpu` with `"The platform you're
compiling for is not supported by winit"` error, should it be launched
at all ?

crates/egui/src/response.rs

@@ -770,9 +770,8 @@ impl Response {
     /// ```
     ///
     /// See also: [`Ui::menu_button`] and [`Ui::close_menu`].
-    pub fn context_menu(self, add_contents: impl FnOnce(&mut Ui)) -> Self {
-        menu::context_menu(&self, add_contents);
-        self
+    pub fn context_menu(&self, add_contents: impl FnOnce(&mut Ui)) -> Option<InnerResponse<()>> {
+        menu::context_menu(self, add_contents)
     }
 }