Add a Dowload button to images and files attached
Currently files and images are open with xdg-open and the media is downloaded in the cache directory. We should provide a way to manage this correctly.
The first step can be to add a download button and to use the FileChooserDialog to ask to the user where to download the file.
http://gtk-rs.org/docs/gtk/struct.FileChooserDialog.html
This is related to: