[OSX] Refactor Native App Menu. Move default menu initialization code from ObjC to C#.

[Mikolaytis]

What does the pull request do?

• refactoring of default native app menu initialization code. Moved from native lib to C#.
• Exposed three macOS commands - hide, show all, hide other over INativeApplicationCommands.
• Implemented DisableDefaultApplicationMenuItems in C# side.

What is the current behavior?

MacOS default menu initialization is hard-coded in native lib. No ability to localize. No ability to use mac specific commands.

What is the updated/expected behavior with this PR?

Any user now can add their own app menu in macOS including default macOS specific menu commands.

Checklist

• [* ] Added unit tests (if possible)?
• [* ] Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/Documentation with user documentation

Fixed issues

#6903

[Mikolaytis]

After this PR our app menu looks like:

<NativeMenuItem Command="{Binding ...}"
				Gesture="{Gesture About}"
				Header="{Loc MainMenu_AboutLunacy}" />
<NativeMenuItemSeparator />
<NativeMenuItem Command="{Binding ...}" Header="{Loc MainMenu_CheckForUpdates}" />
<NativeMenuItemSeparator />
<NativeMenuItem Header="{Loc MainMenu_Services}">
	<NativeMenu IsServicesSubmenu="True" />
</NativeMenuItem>
<NativeMenuItemSeparator />
<NativeMenuItem Command="{x:Static controls:AvaloniaCommands.HideAppCommand}" Header="{Loc MainMenu_HideLunacy}" Gesture="Meta+H" />
<NativeMenuItem Command="{x:Static controls:AvaloniaCommands.HideOthersCommand}" Header="{Loc MainMenu_HideOthers}" Gesture="Meta+Alt+H" />
<NativeMenuItem Command="{x:Static controls:AvaloniaCommands.ShowAllCommand}" Header="{Loc MainMenu_ShowAll}" />
<NativeMenuItemSeparator />
<NativeMenuItem Command="{Binding ...}"
				Gesture="{Gesture Quit}"
				Header="{Loc MainMenu_QuitLunacy}" />

Notice: IsServicesSubmenu usage.

public static class AvaloniaCommands
{
    public static ICommand HideAppCommand = new RelayCommand(() =>
        AvaloniaLocator.Current.GetService<INativeApplicationCommands>().HideApp());
    public static ICommand HideOthersCommand = new RelayCommand(() =>
        AvaloniaLocator.Current.GetService<INativeApplicationCommands>().HideOthers());
    public static ICommand ShowAllCommand = new RelayCommand(() =>
        AvaloniaLocator.Current.GetService<INativeApplicationCommands>().ShowAll());
}

[kekekeks]

@danwalmsley

[kekekeks]

I haven't looked at the code diff, but I see some problems just from looking at the API:

The problem with the API is that it exposes properties in a way that doesn't indicate them being OSX-specific. While it's not generally a problem for the main app menu since we only use it for OSX, NativeMenuItem itself is also used for Linux global menu and for TrayIcon across all platforms.

So I'd suggest to rename AvaloniaCommands to MacOSNativeMenuCommands and make IsServicesSubmenu to be an attached property provided by that class.