Files field: enhanced interactions#173

It would be great, if the files field allowed for additional interactions:

1. Adding a file via drag and drop, behaving like a file section in this regard.
2. Adding key shortcuts to either add an existing or upload a new file, like ctlr+click for the one and shift+click for the other.

Both would improve the workflow when editing a lot of pages in a row, saving the user time.

Drag & drop uploads are coming. It would be great if we could turn the keyboard shortcuts into a separate ticket.

Shortcuts moved to https://kirby.nolt.io/330