Add Javascript and HTML support to webview

[mdsw]

It looks like the webview component uses the system webview in a manner similar to https://github.com/webview/webview . Would it be possible to extend the nappgui webview API to support:

1. Injecting Javascript and exposing application functions to the webview (webview_eval, webview_bind, webview_unbind, webview_return)
2. Directly setting the HTML content for the webview from a string (webview_set_html)