Start rebuilding with a cleanly-separated UI framework

gpui/src/platform/mod.rs

@@ -0,0 +1,20 @@
+mod event;
+#[cfg(target_os = "macos")]
+pub mod mac;
+pub mod current {
+    #[cfg(target_os = "macos")]
+    pub use super::mac::*;
+}
+
+use std::path::PathBuf;
+
+use event::Event;
+
+pub trait App {
+    fn on_finish_launching<F: 'static + FnOnce()>(self, callback: F) -> Self where;
+    fn on_become_active<F: 'static + FnMut()>(self, callback: F) -> Self;
+    fn on_resign_active<F: 'static + FnMut()>(self, callback: F) -> Self;
+    fn on_event<F: 'static + FnMut(Event) -> bool>(self, callback: F) -> Self;
+    fn on_open_files<F: 'static + FnMut(Vec<PathBuf>)>(self, callback: F) -> Self;
+    fn run(self);
+}