Images that are attached to messages, either sent or received
can be opened in a new tab by clicking on them.
The previous approach that used Anchors to open the image
attachmets failed in various systems because:
- Chrome on Windows recognised "blob" as protocol and tried
to find an app for it
- Chromium on Ubuntu didn't open a new window to load the URL
The new approach adds a "click" listener to the IMG element and
opens the link using window.open (which seems to be working globaly).
Resolves: #252
49585c8c57