Simulcast, Screen sharing & Various improvements (#4)

* Respect `RTCIceTransportPolicy` enum and organize

* Simplify syntax where possible etc.

* Combine `VideoPreset` and `VideoPresets`

* Default values for `ConnectOptions`

* Build URI instead of String manipulation

* Slight modifications to Exception

* `LiveKitTheme` for example

* `VideoEncoding` class

* Organize imports

* First simulcast implementation

* Remove unnecessary try-catches

* Update Android settings

* Remember uri and token

* example improvements

* `fit` parameter for VideoTrackRenderer

* Simulcast option for example

* Pass `defaultPublishOptions`

* Show only `VideoQuality`

* Pass tests

* Better buildUri logic

* Named parameter to positional

* Explicit imports

* `VideoParameter` instead of `VideoPreset`

* Use `mediaTrack.getSettings` when possible

* Safer dispose logic

* Safer `PCTransport`

Update transport.dart

* Synchronized events for `SignalClient`

* Use logger instead of print

* Make example compile for iOS

* First screen share implementation

* Make example work with screen share

* Example improvement

* Code optimization

* Don't depend on web_socket_channel

* Fix: Unpublish track bug

* Show participant mute state & identity

* Update protos

* Remote mute/unmute

* iOS Background mode

* Separate `createCameraTrack` and `createScreenTrack`

* Clean up

* PB fix

* format

* Fix analyzer warning

* Android clean up

* Update README.md

* Clean up

example/lib/exts.dart

@@ -0,0 +1,18 @@
+import 'package:flutter/material.dart';
+
+extension LKExampleExt on BuildContext {
+  //
+  Future<void> showErrorDialog(dynamic exception) => showDialog<void>(
+        context: this,
+        builder: (ctx) => AlertDialog(
+          title: const Text('Error'),
+          content: Text(exception.toString()),
+          actions: [
+            TextButton(
+              onPressed: () => Navigator.pop(ctx),
+              child: const Text('OK'),
+            )
+          ],
+        ),
+      );
+}