Core: make Gtk app uniqueness optional

Add optional parameter to MauiGtkApplication constructor that
determines if application can have several instances running
at the same time. Default is false (no uniqueness check),
which requires no DBus connections, unlike previous default
(uniqueness is enforced).

src/Core/src/Platform/Gtk/MauiGtkApplication.cs

@@ -10,6 +10,12 @@ namespace Microsoft.Maui
 {
 	public abstract class MauiGtkApplication : IPlatformApplication
 	{
+		/// <param name="enforceUniqueness">If true, only one instance of application can run at any moment.</param>
+		public MauiGtkApplication(bool enforceUniqueness = false)
+		{
+			EnforceUniqueness = enforceUniqueness;
+		}
+
 		protected abstract MauiApp CreateMauiApp();
 
 		static readonly Regex InvalidGtkApplicationIdElementCharRegex = new Regex("[^A-Za-z0-9_\\-]");
@@ -53,6 +59,7 @@ public string? Name
 		public IServiceProvider Services { get; protected set; } = null!;
 
 		public IApplication Application { get; protected set; } = null!;
+		public bool EnforceUniqueness { get; }
 
 		public void Run(params string[] args)
 		{
@@ -135,7 +142,8 @@ protected void StartupLauch(object sender, EventArgs args)
 		protected void Launch(EventArgs args)
 		{
 			Gtk.Application.Init();
-			var app = new Gtk.Application(ApplicationId, GLib.ApplicationFlags.None);
+			var flags = EnforceUniqueness ? GLib.ApplicationFlags.None : GLib.ApplicationFlags.NonUnique;
+			var app = new Gtk.Application(ApplicationId, flags);
 
 			RegisterLifecycleEvents(app);