Expose several internal fields and add factory functions

[coderforlife]

This works to address #181.

Changes:

• An expect/actual set of typealiases is made for each platform defining a NativeWebView as either android.webkit.WebView, WKWebView, or KCEFBrowser.
• The webView field in each platform's IWebView is publicly exposed as a NativeWebView
• WebViewState has a new field nativeWebView which publicly exposes this in the state without exposing anything else
• Make onCreate and onDispose callbacks of WebView() composable take a NativeWebView argument
  • Minor backwards incompatibility is introduced with this if function references are passed as these callbacks instead of lambda/anonymous functions (which will automatically adapt to and ignore the added argument). Old code may need to place { } around the callback call to make it work again.
• Add a factory function to the WebView() composable that allows the caller to specify how web view objects should be created.
  • Minor backwards incompatibility in that if there is any code that assumes the final argument of the WebView() composable is the onDispose callback and moved the lambda out of the paratheses will have to move it into the paratheses.

Both of the minor issues could be mitigated with creating a wrapper function that matches the old version. If you want this added, I can add it to the PR.

[coderforlife]

Please review this PR and hopefully create a new release! Thanks.

[KevinnZou]

@coderforlife Thanks for your contribution! I will review it this week and give you feedback! Thanks!

[KevinnZou]

Both of the minor issues could be mitigated with creating a wrapper function that matches the old version. If you want this added, I can add it to the PR.

@coderforlife Great jobs! Could you please add the wrapper function to match the old version? Thank you!

[coderforlife]

Changes complete.

[KevinnZou]

Great Jobs!