add read_image api to ui.clipboard

[liunux4odoo]

make it easy to read copied image from client clipboard.

[falkoschindler]

Thanks for the pull request, @liunux4odoo!

• To be honest, I don't understand how to use this feature. How can I copy an image to the clipboard? Whatever I try, your demo says I need to copy an image first.

Apart from that, there are some problems with this implementation:

1. We can't simply break existing API by renaming read and write. I suggest to leave them unchanged while adding read_image.
2. The demo doesn't work for two reasons:
   1. The call to ui.clipboard.read_image() needs to be awaited.
   2. Awaiting a JavaScript command isn't allowed on the auto-index client, so the demo needs to be wrapped in a page function. But in order for the demo to keep working when embedded on the NiceGUI website, we need to prevent the decorator from adding a new route. Here is an example how to do that.

[liunux4odoo]

I have reversed the clipboard.read & clipboard.write APIs.
User must copy an image to the clipboad before he can read it.

[falkoschindler]

Ok, I managed to copy an image like this:

1. Open an image in Preview (macOS)
2. Use Edit > Copy

[falkoschindler]

Thanks again, @liunux4odoo!
I just improved the way PIL is imported because it is an optional dependency.
Let's merge!