Send files via DnD
It should be possible to send files by dropping them onto the history. The workflow I imagine is that you drag the file over the window, and a receive feedback that the file will be sent. If the file is then dropped in the designated area, a modal window with a preview of the file being sent is shown (e.g. if it's shown as a thumbnail, for other file types we could maybe use the MIME type icons).
The icon used in the mockup in the preview is "document-send-symbolic".
- add preview when dragging file over window
- add modal window with file preview to confirm sending file
- send file
- preview works as expected
- modal window works as expected
- sending file works as expected