Follow up of zed-industries#33334

After chatting with @MrSubidubi we found out that he had an old defaults
setting (most likely from when he encountered a previous window tabbing
bug):
```
❯ defaults read dev.zed.Zed-Nightly
{
    NSNavPanelExpandedSizeForOpenMode = "{800, 448}";
    NSNavPanelExpandedSizeForSaveMode = "{800, 448}";
    NSNavPanelExpandedStateForSaveMode = 1;
    NSOSPLastRootDirectory = {length = 828, bytes = 0x626f6f6b 3c030000 00000410 30000000 ... dc010000 00000000 };
    "NSWindow Frame NSNavPanelAutosaveName" = "557 1726 800 448 -323 982 2560 1440 ";
    "NSWindowTabbingShoudShowTabBarKey-GPUIWindow-GPUIWindow-(null)-HT-FS" = 1;
}
```

> That suffix is AppKit’s fallback autosave name when no tabbing
identifier is set. It encodes the NSWindow subclass (GPUIWindow), plus
traits like HT (hidden titlebar) and FS (fullscreen).

Which explains why it only happened on the Nightly build, since each
bundle has it's own defaults. It also explains why the tabbar would
disappear when he activated the `use_system_window_tabs` setting,
because with that setting activated, the tabbing identifier becomes
"zed" (instead of the default one when omitted) for which he didn't have
the `NSWindowTabbingShoudShowTabBarKey` default.

The original implementation was perhaps a bit naive and relied fully on
macOS to determine if the tabbar should be shown. I've updated the code
to always hide the tabbar, if the setting is turned off and there is
only 1 tab entry.

While testing, I also noticed that the menu's like 'merge all windows'
wouldn't become active when the setting was turned on, only after a full
workspace reload. So I added a setting observer as well, to immediately
set the correct window properties to enable all the features without a
reload.

Release Notes:

- N/A