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>

Cargo.lock

@@ -1614,6 +1614,26 @@ dependencies = [
  "pin-project-lite",
 ]
 
+[[package]]
+name = "external_eventloop"
+version = "0.1.0"
+dependencies = [
+ "eframe",
+ "env_logger",
+ "winit",
+]
+
+[[package]]
+name = "external_eventloop_async"
+version = "0.1.0"
+dependencies = [
+ "eframe",
+ "env_logger",
+ "log",
+ "tokio",
+ "winit",
+]
+
 [[package]]
 name = "fancy-regex"
 version = "0.11.0"
@@ -2446,9 +2466,9 @@ checksum = "bbd2bcb4c963f2ddae06a2efc7e9f3591312473c50c6685e1f298068316e66fe"
 
 [[package]]
 name = "libc"
-version = "0.2.161"
+version = "0.2.168"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8e9489c2807c139ffd9c1794f4af0ebe86a828db53ecdc7fea2111d0fed085d1"
+checksum = "5aaeb2981e0606ca11d79718f8bb01164f1d6ed75080182d3abf017e6d244b6d"
 
 [[package]]
 name = "libloading"
@@ -2603,6 +2623,17 @@ version = "0.5.9"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e53debba6bda7a793e5f99b8dacf19e626084f525f7829104ba9898f367d85ff"
 
+[[package]]
+name = "mio"
+version = "1.0.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2886843bf800fba2e3377cff24abf6379b4c4d5c6681eaf9ea5b0d15090450bd"
+dependencies = [
+ "libc",
+ "wasi 0.11.0+wasi-snapshot-preview1",
+ "windows-sys 0.52.0",
+]
+
 [[package]]
 name = "multiple_viewports"
 version = "0.1.0"
@@ -3860,6 +3891,16 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "socket2"
+version = "0.5.8"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c970269d99b64e60ec3bd6ad27270092a5394c4e309314b18ae3fe575695fbe8"
+dependencies = [
+ "libc",
+ "windows-sys 0.52.0",
+]
+
 [[package]]
 name = "spirv"
 version = "0.3.0+sdk-1.3.268.0"
@@ -4178,6 +4219,20 @@ version = "0.1.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1f3ccbac311fea05f86f61904b462b55fb3df8837a366dfc601a0161d0532f20"
 
+[[package]]
+name = "tokio"
+version = "1.44.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e6b88822cbe49de4185e3a4cbf8321dd487cf5fe0c5c65695fef6346371e9c48"
+dependencies = [
+ "backtrace",
+ "libc",
+ "mio",
+ "pin-project-lite",
+ "socket2",
+ "windows-sys 0.52.0",
+]
+
 [[package]]
 name = "toml_datetime"
 version = "0.6.8"