tray: order unconfigured items alphabetically, log item keys

[Carltoffel]

Closes #4162.

Two gaps around tray ordering.

Ordering is only deterministic for configured items

orders pins the items you name. Everything else keeps the order in which the
applications happened to win the race to register on D-Bus, which is what #4162
is about — it differs between restarts, and as noted in that thread it differs
between the outputs of a single bar, so two monitors show two different trays.

To get a stable layout today you have to enumerate every item with an explicit
integer, including the ones you have no opinion about.

This sorts items sharing an order value by their key instead of leaving them at
insertion order. Items with no entry in orders all sit at 0, so they end up
alphabetical between the negative and the positive ones, and the default layout
is stable without any configuration at all.

The key an item is addressed by is invisible

orders and icons address an item by its SNI Id, or by its lowercased
tooltip for Chrome-based apps whose Id is not unique. Neither appears anywhere
in the UI, and the man page only said "name of item" — so configuring Rocket.Chat
means guessing Rocket.Chat_status_icon_1.

Log it once per key at info level:

[info] tray: item key 'nm-applet'
[info] tray: item key 'Rocket.Chat_status_icon_1'

Deduplicated on purpose: every bar runs its own Host with its own Item
objects, so an unguarded line fires once per output, and again on every Id
property update.

The man page gets a section documenting both the keys and the default order.

Notes

Item::sort_key holds the key separately from the icon id. The two are only
incidentally the same string — the icon id is what gets looked up in icons,
and tying the sort order to it would mean an icon config change silently
reshuffles the tray.

No new config options.

Testing

Against a live tray with six items (gammastep, nm-applet, Telegram, and three
Electron apps):

• no orders at all → Claude_status_icon_1, gammastep, nm-applet,
  Rocket.Chat_status_icon_1, TelegramDesktop, vesktop_status_icon_1
• {"gammastep": -5, "nm-applet": 10, "Claude_status_icon_1": 20} → gammastep,
  then the alphabetical remainder, then nm-applet, then Claude
• each key logged exactly once on a two-output setup (twice before this change)