Show remote images shortcut
This addresses #750 :)
First, I did the exact same way mjog described it. This led to (at least for me) unexpected behavior as selecting a conversation and pressing CTRL+I did nothing because the conversation wasn't focused. I decided to invoke it from the MainWindow because of that reason.
Also while testing I thought the user expects only toggling the revealed messages as only these also got the "show remote images"-bar.
Looking forward to get some feedback!
- Tim