grouped-window-list: Fix store disconnect() skipping listeners

[somedev7]

Problem

createStore().disconnect(id) iterates forward over listeners and splice()s every match. One connect({a, b}) registers two listeners with the same id. After the first one is removed, the second shifts into index i and the loop skips it, so the listener for the second key is never removed.

WindowThumbnail connects {scrollActive, thumbnailCloseButtonOffset} to the applet-wide state, so every destroyed thumbnail stays referenced from the applet's listener list forever: the thumbnailCloseButtonOffset closure keeps the thumbnail and everything it references alive. The same happens to other multi-key connect() calls on groupState for as long as the app group lives.

Impact

Found while investigating periodic desktop stutter on Linux Mint 22.3 / Cinnamon 6.6.9 / cjs 115.1. After 33 days of uptime the live JS heap (imports.system.dumpMemoryInfo() → gcBytes) was 329 MB. Since cjs schedules a non-incremental "big hammer" GC on toggle-down, which on a live desktop fires every ~10 s, the compositor was blocked 200–234 ms every 10 s (a forced imports.system.gc() took 215 ms). After restarting Cinnamon: 6 MB heap, 7 ms GC. A heap dump showed several leaks; the biggest one is #13956 (fix in #13958), this is another one.

The heap dump after 33 days had 4,246 live thumbnailCloseButtonOffset closures, all held through the store's listener list.

Reproduce

In Looking Glass:

let ap = imports.ui.appletManager.getRunningInstancesForUuid('grouped-window-list@cinnamon.org')[0];
let fired = 0;
let id = ap.state.connect({testA: () => fired++, testB: () => fired++});
ap.state.disconnect(id);
ap.state.set({testA: 1, testB: 1});
fired  // 1 without the fix, 0 with it

Fix

Iterate backwards. Verified on 6.6.9 by running the applet with this change: the snippet above returns 0, and the window list works normally.

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