Added Context::is_context_menu_open()

[dmlary]

I encountered a case where I needed to know if an egui context menu was open, even if the mouse cursor was not in an egui area (full details in discord: https://discord.com/channels/900275882684477440/1142659953102966884).

I found that every resource I needed to detect that the context menu was open was either private, or pub(crate). Also, the Response returned by Response::context_menu() contains no way to detect that the context menu was opened or closed, so there's nothing to hook there.

While it is possible to wrap the Response::context_menu() in code to do state-tracking, it becomes necessary to duplicate that code in every place you call context_menu().

In this commit, I add Context::is_context_menu_open(). Named similarly to Context::is_pointer_over_area(), this method will return true if any context menu is open. This is possible because the context menu uses a temp var with a fixed Id to store its state. This method just fetches the state, and interrogates it to see if there is a menu present.

One pub(crate) helper method, BarState::has_root(), was added as all the fields needed to perform the check were private to the menu module.

I've also updated the Context Menu demo to show the result of is_context_menu_open() to verify the code functions as expected.

[dmlary]

Video of updated demo; github wouldn't inline it in the description.

Screen.Recording.2023-08-20.at.2.10.26.PM.mov

[dmlary]

And here's the equivalent code needed to track the context_menu state from outside of egui:

        // create an area covering the entire screen that we use to detect
        // right-click to open the context menu.
        egui::Area::new("context_menu_area")
            // set the area as background so 'is_pointer_over_area()' ignores
            // this area when checking the pointer location.
            .order(egui::Order::Background)
            // set the position to (0, 0) to override padding/spacing and cover
            // the entire screen
            .fixed_pos([0.0, 0.0])
            .show(ctx, |ui| {
                // We need to know when the context menu is open so we don't
                // move the tile-selection cursor.  There is no mechanism in
                // egui to do this, so instead we need to manually track it
                // ourselves.
                //
                // Assume the menu is always closed; update it to open when it
                // is drawn.  If the menu state doesn't match the UiState value
                // at the end, update the UiState.
                let mut menu_is_open = false;

                // create a response covering the entire screen to detect click
                // for the context menu
                ui.allocate_response(ui.available_size(), egui::Sense::click())
                    .context_menu(|ui| {
                        let mut close_menu = false;
                        if ui.button("Link to new Chunk").clicked() {
                            close_menu = true;
                        }
                        if ui.button("Link to Existing Chunk").clicked() {
                            close_menu = true;
                        }
                        if ui.button("Link to Existing Tile").clicked() {
                            close_menu = true;
                        }

                        if close_menu {
                            ui.close_menu();
                        } else {
                            menu_is_open = true;
                        }
                    });

                // if the menu state changed, upsdate the ui state
                if menu_is_open != ui_state.context_menu {
                    debug!("context menu state change: {:?}", menu_is_open);
                    world.resource_mut::<UiState>().context_menu = menu_is_open;
                }
            });