Upgrade deps/webview to v0.10.0

[rainbyte]

Recently webview/webview was rewritten, and this PR syncs webviewhs with that version.

Interesting changes:

• Main loop in handled exclusively by webview in an efficient way (avoids high cpu usage).
• Supports binding multiple callbacks associated to a request id (invoke is now internal).
• Supports sending back results to frontend in an request-response fashion.

Some things are not supported anymore by upstream, so they were removed:

• webview_set_fullscreen
• webview_set_color
• webview_dialog
• webview_debug
• webview_print_log

Those can be implemented in JavaScript or via system specific backends.

It is published as a draft to discuss how to improve implementation.

TODO:

• Docs: describe how to bind multiple callbacks
• Docs: describe request-response mechanism
• Improve inline documentation
• Port webviewhs/examples
• Test webviewhs/examples-light