Improve dev-experience on setting adapter preferences

[maxkatz6]

Currently, WebView adapter system is not flexible enough, when developer has preference in backend prioritiies. For example, whether it should be WPE and GTK if both are available, or Win.WebView1 or Win.WebView2.
For normal views it can be configured with environemnt requeted events. But it's not configurable for dialogs, for example, at least on Linux.
Additional problem adds offscreen or not selection, on applicable backends, for example WPE is always offscreen. but GTK and WebView2 are optionally offscreen.

[mjcheetham]

Now that #77 has been merged, is this issue addressed or are there more pieces needed?

Also just wondering the release schedules for the Avn.Controls.WV package - no rush, just wish to plan when this feature is likely to make it out the door.