Support overlays on top of Qt WebView on iOS and Android

There is a demand to enable adding UI elements and graphical operations (in short here "overlay") on top of WebView which uses native web components on iOS and Android. Note that other Qt platforms are not listed here since the assumption is that the user can use an overlay in WebView since it is using QtWebEngine and so there an access to the surface on top of the web content.

There were different prototypes and trials made in the past to implement this. Some worked not so well, some better. The technology progresses and it would make sense to look into this now to address this need.

I post this as epic along with two linked tasks for iOS and Android for better tracking. Feel free to comment or provide any other feedback.