add a keyboard shortcut to go to an unread room

Pressing alt+A switches to a room with

• a mention (actually a notification count)
• or any unread message (a room in bold)
• and distinct from the current one

In case rooms with mentions and unread messages are both present, mentions are preferred.

this enables the following workflow: repeatedly pressing alt+A visits all channels with something to be read, starting with mentions.

This feature is present in weechat and nheko.

Note about the keyboard shortcut: it was chosen to be identical to weechat and nheko. I think A means "activity".