Add external eventloop support (#6750)

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* Closes #2875
* Closes https://github.com/emilk/egui/pull/3340
* [x] I have followed the instructions in the PR template

Adds `create_native`. Similiar to `run_native` but it returns an
`EframeWinitApplication` which is a `winit::ApplicationHandler`. This
can be run on your own event loop. A helper fn `pump_eframe_app` is
provided to pump the event loop and get the control flow state back.

I have been using this approach for a few months.

---------

Co-authored-by: Will Brown <opensource@rebeagle.com>

examples/external_eventloop_async/src/main.rs

@@ -0,0 +1,15 @@
+#![allow(rustdoc::missing_crate_level_docs)] // it's an example
+
+#[cfg(target_os = "linux")]
+mod app;
+
+#[cfg(target_os = "linux")]
+fn main() -> std::io::Result<()> {
+    app::run()
+}
+
+// Do not check `app` on unsupported platforms when check "--all-features" is used in CI.
+#[cfg(not(target_os = "linux"))]
+fn main() {
+    println!("This example only supports Linux.");
+}