Add shared QML application menu and action proxies

[xARSENICx]

• Adds a shared native QML MenuBar with File, Library, View, Options, Developer, and Help menus on macOS.
• Adds reusable QML command handling for track loading, full screen, About, Keywheel, and related dialogs.
• Exposes application state, configured shortcuts, developer actions, and settings metadata through QmlApplicationProxy.
• Exposes library rescan, export, search, playlist, crate, and Auto DJ actions through QmlLibraryProxy, including a QML scan-summary dialog.
• Exposes URL-based deck loading and file-dialog metadata through QmlPlayerManagerProxy.
• Creates the conditional Developer menu only after component completion, avoiding startup-time popup activation.
• Keeps the native menu bar macOS-only so QML skins can provide their own in-window menu presentation on Windows and Linux.

[JoergAtGithub]

Suggested change

	Action {
	checkable: true
	checked: vinylDeck3Control.value > 0
	shortcut: Mixxx.Application.menuShortcut("OptionsMenu_EnableVinyl3", "Ctrl+U")
	text: qsTr("Enable Vinyl Control 3")
	onTriggered: vinylDeck3Control.value = vinylDeck3Control.value > 0 ? 0.0 : 1.0
	}
	Action {
	enabled: root.numberOfDecks >= 3
	checkable: true
	checked: vinylDeck3Control.value > 0
	shortcut: Mixxx.Application.menuShortcut("OptionsMenu_EnableVinyl3", "Ctrl+U")
	text: qsTr("Enable Vinyl Control 3")
	onTriggered: vinylDeck3Control.value = vinylDeck3Control.value > 0 ? 0.0 : 1.0
	}

Same as for the File menu Load to deck menu entries

[JoergAtGithub]

This is not working for me, always all 4 menu actions are enabled - maybe, because it's a nested sub-menu?

[JoergAtGithub]

The last commit does not change the behaviour. Allways all 4 menu entries are enabled. I guess that all these variables are out of scope of this nested submenu.