debugger: Add actions and keybindings for opening the thread and session menus (#31135)

Makes it possible to open and navigate these menus from the keyboard.

I also removed the eager previewing behavior for the thread picker,
which was buggy and came with a jarring layout shift.

Release Notes:

- Debugger Beta: Added the `debugger: open thread picker` and `debugger:
open session picker` actions.

crates/ui/src/components/dropdown_menu.rs

@@ -2,6 +2,8 @@ use gpui::{ClickEvent, Corner, CursorStyle, Entity, Hsla, MouseButton};
 
 use crate::{ContextMenu, PopoverMenu, prelude::*};
 
+use super::PopoverMenuHandle;
+
 #[derive(Debug, Default, Clone, Copy, PartialEq, Eq, Hash)]
 pub enum DropdownStyle {
     #[default]
@@ -22,6 +24,7 @@ pub struct DropdownMenu {
     menu: Entity<ContextMenu>,
     full_width: bool,
     disabled: bool,
+    handle: Option<PopoverMenuHandle<ContextMenu>>,
 }
 
 impl DropdownMenu {
@@ -37,6 +40,7 @@ impl DropdownMenu {
             menu,
             full_width: false,
             disabled: false,
+            handle: None,
         }
     }
 
@@ -52,6 +56,7 @@ impl DropdownMenu {
             menu,
             full_width: false,
             disabled: false,
+            handle: None,
         }
     }
 
@@ -64,6 +69,11 @@ impl DropdownMenu {
         self.full_width = full_width;
         self
     }
+
+    pub fn handle(mut self, handle: PopoverMenuHandle<ContextMenu>) -> Self {
+        self.handle = Some(handle);
+        self
+    }
 }
 
 impl Disableable for DropdownMenu {
@@ -85,6 +95,7 @@ impl RenderOnce for DropdownMenu {
                     .style(self.style),
             )
             .attach(Corner::BottomLeft)
+            .when_some(self.handle.clone(), |el, handle| el.with_handle(handle))
     }
 }
 
@@ -159,17 +170,11 @@ pub struct DropdownTriggerStyle {
 impl DropdownTriggerStyle {
     pub fn for_style(style: DropdownStyle, cx: &App) -> Self {
         let colors = cx.theme().colors();
-
-        if style == DropdownStyle::Solid {
-            Self {
-                // why is this editor_background?
-                bg: colors.editor_background,
-            }
-        } else {
-            Self {
-                bg: colors.ghost_element_background,
-            }
-        }
+        let bg = match style {
+            DropdownStyle::Solid => colors.editor_background,
+            DropdownStyle::Ghost => colors.ghost_element_background,
+        };
+        Self { bg }
     }
 }