feature: dynamic members window

[Nemo157]

I'm playing around with some layout ideas at the moment, and one thing I think would be very useful is a window that dynamically shows the members of the last focused room, e.g. I have a layout like this with three channels I interact with a lot and a channel list to see unreads in other channels (which I then open over top one of the secondary channels):

┌DMs & Rooms─┐┌Channel 1┐┌Channel 2┐
│            ││         ││         │
│            ││         ││         │
│            ││         ││         │
└────────────┘│         │└─────────┘
┌Room Members┐│         │┌Channel 3┐
│            ││         ││         │
│            ││         ││         │
└────────────┘└─────────┘└─────────┘

Currently the Room Members just shows the members of the main Channel 1, but I would like to easily see the members of different rooms as I interact with them.