Make egu::menu types public (#4544)

Sometimes we need to fix the current state of the application in the
moment when you open the context menu, to save it, and to use it during
creation of context menu and response handling. Making some structs,
related with menu creating, allow us to create functions for this cases.
For example,
```rust
pub fn context_menu_custom<'a, T>(
    response: &Response,
    //variable for fixing state in the moment when you open context menu
    state: &mut T,
    //function which allow to get some king of state.
    //In this case state depends on cursor position, in other cases it may depend on system time or something else
    get_state: impl FnOnce(Pos2) -> T,
    //set contents of menu depending on state
    set_contents: impl 'a + FnOnce(&T) -> Box<dyn 'a + FnOnce(&mut Ui)>,
) -> Option<InnerResponse<()>> {
    let menu_id = Id::new("__egui::context_menu");
    let mut bar_state = BarState::load(&response.ctx, menu_id);
    let root = &mut bar_state;

    let menu_response = MenuRoot::context_interaction(response, root);
    if let egui::menu::MenuResponse::Create(p, _) = &menu_response {
        *state = get_state(*p);
    };

    let add_contents = set_contents(&state);

    MenuRoot::handle_menu_response(root, menu_response);
    let inner_response = bar_state.show(response, add_contents);

    bar_state.store(&response.ctx, menu_id);
    inner_response
}
```
The example of using such function you may see in [`my
repository`](https://github.com/sor-ca/context_menu)
It is very simple example, and is this case, the problem may be solved
without fn context_menu_custom, but in more complex situations, it may
be very useful
Related issue: #4162

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---------

Co-authored-by: Emil Ernerfeldt <[email protected]>

crates/egui/src/menu.rs

@@ -25,16 +25,16 @@ use std::sync::Arc;
 
 /// What is saved between frames.
 #[derive(Clone, Default)]
-pub(crate) struct BarState {
+pub struct BarState {
     open_menu: MenuRootManager,
 }
 
 impl BarState {
-    fn load(ctx: &Context, bar_id: Id) -> Self {
+    pub fn load(ctx: &Context, bar_id: Id) -> Self {
         ctx.data_mut(|d| d.get_temp::<Self>(bar_id).unwrap_or_default())
     }
 
-    fn store(self, ctx: &Context, bar_id: Id) {
+    pub fn store(self, ctx: &Context, bar_id: Id) {
         ctx.data_mut(|d| d.insert_temp(bar_id, self));
     }
 
@@ -242,7 +242,7 @@ pub(crate) fn context_menu_opened(response: &Response) -> bool {
 
 /// Stores the state for the context menu.
 #[derive(Clone, Default)]
-pub(crate) struct MenuRootManager {
+pub struct MenuRootManager {
     inner: Option<MenuRoot>,
 }
 
@@ -287,7 +287,7 @@ impl std::ops::DerefMut for MenuRootManager {
 
 /// Menu root associated with an Id from a Response
 #[derive(Clone)]
-pub(crate) struct MenuRoot {
+pub struct MenuRoot {
     pub menu_state: Arc<RwLock<MenuState>>,
     pub id: Id,
 }
@@ -372,7 +372,7 @@ impl MenuRoot {
     }
 
     /// Interaction with a context menu (secondary click).
-    fn context_interaction(response: &Response, root: &mut Option<Self>) -> MenuResponse {
+    pub fn context_interaction(response: &Response, root: &mut Option<Self>) -> MenuResponse {
         let response = response.interact(Sense::click());
         let hovered = response.hovered();
         let secondary_clicked = response.secondary_clicked();
@@ -398,7 +398,7 @@ impl MenuRoot {
         })
     }
 
-    fn handle_menu_response(root: &mut MenuRootManager, menu_response: MenuResponse) {
+    pub fn handle_menu_response(root: &mut MenuRootManager, menu_response: MenuResponse) {
         match menu_response {
             MenuResponse::Create(pos, id) => {
                 root.inner = Some(Self::new(pos, id));
@@ -421,8 +421,8 @@ impl MenuRoot {
     }
 }
 
-#[derive(Copy, Clone, PartialEq)]
-pub(crate) enum MenuResponse {
+#[derive(Copy, Clone, PartialEq, Eq)]
+pub enum MenuResponse {
     Close,
     Stay,
     Create(Pos2, Id),
@@ -562,7 +562,10 @@ impl SubMenu {
     }
 }
 
-pub(crate) struct MenuState {
+/// Components of menu state, public for advanced usage.
+///
+/// Usually you don't need to use it directly.
+pub struct MenuState {
     /// The opened sub-menu and its [`Id`]
     sub_menu: Option<(Id, Arc<RwLock<MenuState>>)>,