Re-work event loop run() API so it can return a Result

This re-works the portable `run()` API that consumes the `EventLoop` and
runs the loop on the calling thread until the app exits.

This can be supported across _all_ platforms and compared to the
previous `run() -> !` API is now able to return a `Result` status on all
platforms except iOS and Web. Fixes: #2709

By moving away from `run() -> !` we stop calling `std::process::exit()`
internally as a means to kill the process without returning which means
it's possible to return an exit status and applications can return from
their `main()` function normally.

This also fixes Android support where an Activity runs in a thread but
we can't assume to have full ownership of the process (other services
could be running in separate threads).

Additionally all examples have generally been updated so that `main()`
returns a `Result` from `run()`

Fixes: #2709

examples/window.rs

@@ -10,7 +10,7 @@ use winit::{
 #[path = "util/fill.rs"]
 mod fill;
 
-fn main() {
+fn main() -> Result<(), impl std::error::Error> {
     SimpleLogger::new().init().unwrap();
     let event_loop = EventLoop::new();
 
@@ -37,5 +37,5 @@ fn main() {
             }
             _ => (),
         }
-    });
+    })
 }