FileUpload can handle pasted and dropped files

[tomasherceg]

I needed to add support for pasting or dropping files in textarea, so I refactored the FileUpload core functionality and exposed some of its properties as attached:

    <dot:TextBox Type="MultiLine" Text="{value: Text}"
                 FileUpload.UploadOnPasteOrDrop="{value: Files}"
                 FileUpload.UploadCompleted="{command: OnUploadCompleted()}"/>

When a file is pasted or dropped to the textarea, it will be automatically uploaded and placed to the Files collection.

We'll need to look at the implementation, I don't like it much - I was fighting with TypeScript definitions of DotVVM observables.