feat: Add single-instance mode to prevent multiple instances

[bv-saketha-rama]

Solves #1906

Changes

1. Added Settings Constant (SettingNames.cs)
   internal const string SINGLE_INSTANCE_MODE = "single-instance-mode";

2. Conditional Application Flags (Main.cs)
   `// Check if single-instance mode is enabled
   var settings = new SettingsManager ();
   bool singleInstance = settings.GetSetting (SettingNames.SINGLE_INSTANCE_MODE, false);

// Use appropriate flags based on preference
var flags = singleInstance ? Gio.ApplicationFlags.HandlesOpen
: Gio.ApplicationFlags.NonUnique;
var app = Adw.Application.New (PintaCore.ApplicationId, flags);`

3. OnOpen Event Handler (Main.cs)
   // Handle file opening in existing instance if (singleInstance) { app.OnOpen += (_, args) => { main_window.Activate (); foreach (var file in args.Files) PintaCore.Workspace.OpenFile (file); }; }

Behaviour

• Disabled (default): Multiple windows, backward compatible
• Enabled: All files open as tabs in one window
• Configuration: Edit ~/.config/Pinta/settings.xml:
  <setting name="single-instance-mode" type="System.Boolean">True</setting>

[cameronwhite]

I'm pretty sure this could use PintaCore.Settings instead to avoid creating another SettingsManager (and reloading the settings from disk again)

The line below uses PintaCore.ApplicationId so it should be safe to run the static constructors at this point

[cameronwhite]

I think we'd need to find some way of configuring this setting in the GUI. If it requires editing the xml settings then it may as well not exist for the vast majority of users :)

[bv-saketha-rama]

that's right

[cameronwhite]

Testing on macOS the single-instance mode didn't seem to work, and multiple instances were launched. I'm not sure if this is a GTK limitation ..
Which platform(s) were you testing on?

[bv-saketha-rama]

I tried it on linux system. I will check it again if I made any mistake.