File drag and drop support

[Pastitas]

Is your feature request related to a problem? Please describe.
Right now there's no action associated with dragging and dropping files on kasmvnc webpage, instead the default action by the browser is done. Also in the settings menu there's an option for file upload that, when that specific window is open, allows for file uploads via drag and drop.

Describe the solution you'd like
Add an togglable option to make the app upload automatically any files that are dragged and dropped there in the main page (maybe using some default upload folder setting) .

Describe alternatives you've considered
If this could somehow be integrated with the vnc host to drag the same file to the used application inside would be amazing but that seems way more complex, maybe that could be a future improvement

Additional context
This makes sense for applications in which you'd normally want to drag and drop a file into.