replace UIWebView with WKWebView

[dautermann]

This PR, if you choose to merge it, replaces UIWebView with the way more useful & modern WKWebView. As the first few lines of the WKWebView documentation state:

Important

Starting in iOS 8.0 and OS X 10.10, use WKWebView to add web content to your app. Do not use UIWebView or WebView.

This app uses Cordova iOS 4.3.1 and cordova-plugin-wkwebview-engine.

I also updated cordova-plugin-file.

I did make one change in the bowels of CordovaLib, in CDVViewController.h you'll see:

@property (nonatomic, readonly, strong) WKWebView* webView;

which makes webView a lot more fun and useful to play with than what it was declared as before (a mere UIView).

[ben-ng]

Sweet! I'd love to merge this.

Can you try to get the tests passing on CI? It looks like they're failing because the iPhone simulator version in .travis.yml is too old. Replacing it with iphonesimulator9.3 should work.

[ben-ng]

Also, this PR is far too large for me to review. I'm not actively working on PhoneGap stuff right now, so I've added you as a contributor to the project. Merge when you're happy 👍