Core: better ApplicationId for MauiGtkApplication

Improve algorithm for ApplicationId property in
MauiGtkApplication class. Filter out invalid chars. For Name
property, which is used as last element in ApplicationId, make
default value be executable's filename.

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

@@ -4,23 +4,42 @@
 using Microsoft.Maui.LifecycleEvents;
 using Gtk;
 using Microsoft.Maui.Dispatching;
+using System.Text.RegularExpressions;
 
 namespace Microsoft.Maui
 {
 	public abstract class MauiGtkApplication : IPlatformApplication
 	{
 		protected abstract MauiApp CreateMauiApp();
 
+		static readonly Regex InvalidGtkApplicationIdElementCharRegex = new Regex("[^A-Za-z0-9_\\-]");
+
 		// https://docs.gtk.org/gio/type_func.Application.id_is_valid.html
-		// TODO: find a better algo for id
-		public virtual string ApplicationId => $"{typeof(MauiGtkApplication).Namespace}.{nameof(MauiGtkApplication)}.{Name}".PadRight(255, ' ').Substring(0, 255).Trim();
+		public virtual string ApplicationId
+		{
+			get
+			{
+				var name = InvalidGtkApplicationIdElementCharRegex.Replace(Name!, "_");
+				if (name.Length == 0 || (name[0] >= '0' || name[0] <= '9'))
+					name = "_" + name;
+				return $"{typeof(MauiGtkApplication).Namespace}.{nameof(MauiGtkApplication)}.{name}".PadRight(255, ' ').Substring(0, 255).Trim();
+			}
+		}
 
 		string? _name;
 
 		// https://docs.gtk.org/gio/type_func.Application.id_is_valid.html
 		public string? Name
 		{
-			get => _name ??= $"A{Guid.NewGuid()}";
+			get
+			{
+				if (_name is null)
+				{
+					var exeName = Environment.GetCommandLineArgs()[0];
+					_name = string.IsNullOrEmpty(exeName) ? "Unknown" : System.IO.Path.GetFileName(exeName);
+				}
+				return _name;
+			}
 			set { _name = value; }
 		}