macOS: option to open VM windows as tabs of the main window

[LayerTM]

Addresses #3812.

Many people run one or two VMs and would rather keep the VM screen inside UTM's main window than manage a separate window for each VM. macOS already provides this through native window tabbing, but UTM turns tabbing off app-wide (NSWindow.allowsAutomaticWindowTabbing = false in ContentView).

This adds a setting, Settings → Application → "Open virtual machine windows as tabs of the main window", off by default, so nothing changes for anyone who does not enable it.

When it is on:

• a VM window opened from the library, or through AppleScript / utmctl start, is attached as a tab of the library window (addTabbedWindow(_:ordered:)), since both paths go through UTMData.run(vm:);
• automatic window tabbing is no longer disabled at launch (read at launch, so this part takes effect after a relaunch);
• if the library window is closed, the VM window opens on its own as it does today.

Changes are limited to three files: Platform/Shared/ContentView.swift (one line), Platform/macOS/SettingsView.swift (the toggle), Platform/macOS/UTMDataExtension.swift (the attach step).

Tested on macOS 26 (Apple silicon) with an unsigned build of this branch and a QEMU VM (aarch64, display on):

• start from the library → the VM screen opens as a tab of the main window;
• utmctl start <vm> with UTM already open → same, no separate window;
• three starts in total (library button, AppleScript/utmctl twice), no crashes.

With the setting off, the attach step returns before doing anything and allowsAutomaticWindowTabbing is set to false exactly as before (read from the code; not separately exercised).

The new string is not localized yet.