Implement crude encrypted EventImage downloading

When a message image is clicked in an encrypted room, the full file will
be download (no GUI indication for now, just a print in terminal) and
opened when ready.

Before downloading, copying the image address via right click will put
the full image mxc:// URL in clipboard. After downloading, it will be
the local downloaded image path instead.

src/python/media_cache.py

@@ -209,6 +209,8 @@ class Thumbnail(Media):
         parsed = urlparse(self.mxc)
 
         if self.crypt_dict:
+            # Matrix makes encrypted thumbs only available through the download
+            # end-point, not the thumbnail one
             resp = await self.cache.backend.download(
                 server_name = parsed.netloc,
                 media_id    = parsed.path.lstrip("/"),