Rename PlatformLifecycle to MainThreadPlatform

Don't love it, but at least it's accurate.

Co-Authored-By: Max Brunsfeld <maxbrunsfeld@gmail.com>

gpui/src/platform/mac.rs

@@ -11,12 +11,12 @@ mod window;
 use cocoa::base::{BOOL, NO, YES};
 pub use dispatcher::Dispatcher;
 pub use fonts::FontSystem;
-use platform::{MacLifecycle, MacPlatform};
+use platform::{MacMainThreadPlatform, MacPlatform};
 use std::rc::Rc;
 use window::Window;
 
-pub(crate) fn lifecycle() -> Rc<dyn super::Lifecycle> {
-    Rc::new(MacLifecycle::default())
+pub(crate) fn main_thread_platform() -> Rc<dyn super::MainThreadPlatform> {
+    Rc::new(MacMainThreadPlatform::default())
 }
 
 pub(crate) fn platform() -> Rc<dyn super::Platform> {