add windows theme source and platform icon resolvers

crates/theme/src/source/signal.rs

@@ -0,0 +1,38 @@
+//! Wakes the main-thread resolve pipeline from watcher threads, which hold
+//! no gpui context (the pinned gpui's `AsyncApp` is not `Send`).
+
+use gpui::{AsyncApp, Task};
+
+pub(crate) struct ReapplySignal {
+    sender: async_channel::Sender<()>,
+    _task: Task<()>,
+}
+
+impl ReapplySignal {
+    pub(crate) fn install(async_cx: &AsyncApp) -> Self {
+        let (sender, events) = async_channel::unbounded::<()>();
+        let task = async_cx.update(|cx| {
+            cx.spawn(async move |cx| {
+                while events.recv().await.is_ok() {
+                    cx.update(crate::reapply);
+                }
+            })
+        });
+        Self {
+            sender,
+            _task: task,
+        }
+    }
+
+    /// A clone for a watcher thread to poke the pipeline with.
+    pub(crate) fn sender(&self) -> async_channel::Sender<()> {
+        self.sender.clone()
+    }
+
+    /// Pokes the pipeline from a thread holding a [`Self::sender`] clone.
+    pub(crate) fn poke(sender: &async_channel::Sender<()>) {
+        if let Err(error) = sender.try_send(()) {
+            eprintln!("theme: reapply signal failed: {error}");
+        }
+    }
+}