macOS/iOS: Use WKWebView for wxWebView Implementation

WKWebView replaces
WebView on macOS (available since 10.10, deprecated since 10.14) and
UIWebView on iOS (available since 8.0 deprecated since 10.0).
Using WKWebView ensures future compatiblity and unifies the interface
used for iOS and macOS.

interface/wx/webview.h

@@ -347,12 +347,13 @@ public:
     and under Fedora it is webkitgtk4-devel. All wxWEBVIEW_WEBKIT features are
     supported except for clearing and enabling / disabling the history.
 
-    @par wxWEBVIEW_WEBKIT (OSX)
+    @par wxWEBVIEW_WEBKIT (macOS)
 
     The macOS WebKit backend uses Apple's
-    <a href="http://developer.apple.com/library/mac/#documentation/Cocoa/Reference/WebKit/Classes/WebView_Class/Reference/Reference.html#//apple_ref/doc/uid/20001903">WebView</a>
+    <a href="https://developer.apple.com/documentation/webkit/wkwebview">WKWebView</a>
     class. This backend has full support for custom schemes and virtual file
-    systems.
+    systems on macOS 10.13+. In order to use handlers two-step creation has to be used
+    and RegisterHandler() has to be called before Create().
 
     @section async Asynchronous Notifications
 
@@ -565,6 +566,9 @@ public:
     /**
         Registers a custom scheme handler.
         @param handler A shared pointer to a wxWebHandler.
+        @note On macOS in order to use handlers two-step creation has to be
+              used and RegisterHandler() has to be called before Create().
+              With the other backends it has to be called after Create().
     */
     virtual void RegisterHandler(wxSharedPtr<wxWebViewHandler> handler) = 0;