GPUI custom window prompts (#8980)

This adds a GPUI fallback for window prompts. Linux does not support
this feature by default, so we have to implement it ourselves.

This implementation also makes it possible for GPUI clients to override
the platform prompts with their own implementations.

This is just a first pass. These alerts are not keyboard accessible yet,
does not reflect the prompt level, they're implemented in-window, rather
than as popups, and the whole feature need a pass from a designer.
Regardless, this gets us one step closer to Linux support :)

<img width="650" alt="Screenshot 2024-03-06 at 5 58 08 PM"
src="https://github.com/zed-industries/zed/assets/2280405/972ebb55-fd1f-4066-969c-a87f63b22a6f">

Release Notes:

- N/A

crates/gpui/src/app.rs

@@ -28,14 +28,14 @@ use util::{
     ResultExt,
 };
 
-use crate::WindowAppearance;
 use crate::{
     current_platform, image_cache::ImageCache, init_app_menus, Action, ActionRegistry, Any,
     AnyView, AnyWindowHandle, AppMetadata, AssetSource, BackgroundExecutor, ClipboardItem, Context,
     DispatchPhase, Entity, EventEmitter, ForegroundExecutor, Global, KeyBinding, Keymap, Keystroke,
-    LayoutId, Menu, PathPromptOptions, Pixels, Platform, PlatformDisplay, Point, Render,
-    SharedString, SubscriberSet, Subscription, SvgRenderer, Task, TextStyle, TextStyleRefinement,
-    TextSystem, View, ViewContext, Window, WindowContext, WindowHandle, WindowId,
+    LayoutId, Menu, PathPromptOptions, Pixels, Platform, PlatformDisplay, Point, PromptBuilder,
+    PromptHandle, PromptLevel, Render, RenderablePromptHandle, SharedString, SubscriberSet,
+    Subscription, SvgRenderer, Task, TextStyle, TextStyleRefinement, TextSystem, View, ViewContext,
+    Window, WindowAppearance, WindowContext, WindowHandle, WindowId,
 };
 
 mod async_context;
@@ -242,6 +242,7 @@ pub struct AppContext {
     pub(crate) quit_observers: SubscriberSet<(), QuitHandler>,
     pub(crate) layout_id_buffer: Vec<LayoutId>, // We recycle this memory across layout requests.
     pub(crate) propagate_event: bool,
+    pub(crate) prompt_builder: Option<PromptBuilder>,
 }
 
 impl AppContext {
@@ -301,6 +302,7 @@ impl AppContext {
                 quit_observers: SubscriberSet::new(),
                 layout_id_buffer: Default::default(),
                 propagate_event: true,
+                prompt_builder: Some(PromptBuilder::Default),
             }),
         });
 
@@ -1207,6 +1209,23 @@ impl AppContext {
     pub fn has_active_drag(&self) -> bool {
         self.active_drag.is_some()
     }
+
+    /// Set the prompt renderer for GPUI. This will replace the default or platform specific
+    /// prompts with this custom implementation.
+    pub fn set_prompt_builder(
+        &mut self,
+        renderer: impl Fn(
+                PromptLevel,
+                &str,
+                Option<&str>,
+                &[&str],
+                PromptHandle,
+                &mut WindowContext,
+            ) -> RenderablePromptHandle
+            + 'static,
+    ) {
+        self.prompt_builder = Some(PromptBuilder::Custom(Box::new(renderer)))
+    }
 }
 
 impl Context for AppContext {