CordovaSim: InAppBrowser plugin problems: does not work on Fedora, missing custom scrollbars on Windows

It is quite tricky, what it does:
1. On Mac and Linux it utilizes StackLayout to switch between InAppBrowser and the main browser.
2. On Windows # 1 results in native crashes, so we do not use StackLayout there. Instead we set InAppBrowser as the child of the container of the main browser. The main browser temporarily becomes a child of the InAppBrowser while it is shown. 
3. Also this PR adds custom scroll-bars in InAppBrowser on Windows.