Improve the fast room switcher

[sandhose]

The C^k fast room switcher is a great piece of UI, and can be improved, inspired by what Discord and Slack do.

Slack shows you the channel with the last mentions first. It lets you jump to the next unread room with a few keystrokes. It also pre-selects the first item.

Discord does the same, and also pre-selects the last room visited when the search field is empty. It lets you go back and forth a room rapidly.

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Also, selecting items with the arrow keys (see #3114) seems to have broken recently according to Michael

[lukebarnard1]

We now do up/down navigation of the room list.

Pre-selection is certainly an interesting feature, I can see last channel and most recent mention being both being very useful.

Thinking about it, I mostly click on a very small subset of rooms in my favourites and I wonder how others use their room lists in Riot.

[kiliankoe]

The floating panel UI for a switcher is definitely popular, Telegram (at least the macOS desktop client) also features this, incl. preselection of the first element 👍

[SimonBrandner]

Closing in favour of #19530