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

README.md

@@ -6,7 +6,19 @@ This package is published to pub.dev as [livekit_client](https://pub.dev/package
 
 ## Docs
 
-Docs and guides at [https://docs.livekit.io](https://docs.livekit.io)
+More Docs and guides are available at [https://docs.livekit.io](https://docs.livekit.io)
+
+## Current supported features
+
+| Feature | Subscribe/Publish | Simulcast | Background mode | Screen sharing |
+| :-----: | :---------------: | :-------: | :-------------: | :------------: |
+|   Web   |         🟢         |     🟢     |        🟢        |       🟢        |
+|   iOS   |         🟢         |     🟡     |        🟡        |       🔴        |
+| Android |         🟢         |     🟡     |        🟡        |       🔴        |
+
+🟢 = Available
+🟡 = Coming soon (Work in progress)
+🔴 = Not currently available (Possibly in the future)
 
 ## Installation
 
@@ -23,13 +35,25 @@ dependencies:
 Camera and microphone usage need to be declared in your `Info.plist` file.
 
 ```xml
-...
 <dict>
+  ...
   <key>NSCameraUsageDescription</key>
   <string>$(PRODUCT_NAME) uses your camera</string>
   <key>NSMicrophoneUsageDescription</key>
   <string>$(PRODUCT_NAME) uses your microphone</string>
-</dict>
+```
+
+Your application can still run the voice call when it is switched to the background if the background mode is enabled. Select the app target in Xcode, click the Capabilities tab, enable Background Modes, and check **Audio, AirPlay, and Picture in Picture**.
+
+Your `Info.plist` should have the following entries.
+
+```xml
+<dict>
+  ...
+  <key>UIBackgroundModes</key>
+  <array>
+    <string>audio</string>
+  </array>
 ```
 
 ### Android