Improvements to "Previous/next unread room or DM" shortcut

Current implementation of the shift-ctrl-down button works but could be improved
a lot. IRC clients such as irssi have perfected it.

Current implementation just jumps to the next room with unreads (as the key name says).
On irssi & other better IRC clients there's a key that does about the same but with
prioritized order.

I'd suggest a Matrix adaptation that would work with the following algorithm:

* Jump to next DM with new messages, or
* Jump to next room with highlight, or
* Jump to room with oldest unread messages

Low priority and historical rooms should be ignored in this cycle (currently they
are included). This could be a new button (alt-a is standard in IRC clients) or replace
the functionality of shift-ctrl-down. This is very important power user key as you can
read everything just by hitting the same shortcut.

### Version information

Element Desktop 1.17

The issue does not name files or tests; start by locating the implementation of the Shift-Ctrl-Down unread-room shortcut in the Element Desktop code. Compare its navigation with the proposed priority order and verify that low-priority and historical rooms are skipped; done means the shortcut follows the agreed behavior.

Written by the indexing model from the issue text.