gtk: add gtk-single-instance setting to allow disabling of it

This is based on our conversation on Discord and adds a setting for GTK
that allows disabling the GTK single-instance mode.

If this is off, it's possible to start multiple applications from the
same release binary.

Tested like this:

```
$ zig build -Dapp-runtime=gtk -Doptimize=ReleaseFast && ./zig-out/bin/ghostty --gtk-single-instance=false

[... starts new application ...]
```

and

```
$ zig build -Dapp-runtime=gtk -Doptimize=ReleaseFast && ./zig-out/bin/ghostty --gtk-single-instance=true
info: ghostty version=0.1.0-main+42a22893
info: runtime=apprt.Runtime.gtk
info: font_backend=font.main.Backend.fontconfig_freetype
info: dependency harfbuzz=8.0.0
info: dependency fontconfig=21400
info: renderer=renderer.OpenGL
info: libxev backend=main.Backend.io_uring
info(os): LANG is not valid according to libc, will use en_US.UTF-8
info: reading configuration file path=/home/mrnugget/.config/ghostty/config
info(config): default shell source=env value=/usr/bin/zsh

(process:49045): GLib-GIO-WARNING **: 13:55:56.116: Your application did not unregister from D-Bus before destruction. Consider using g_application_run().

[exits]
```

1 files changed, 9 insertions, 0 deletions

diff --git a/src/config.zig b/src/config.zig
index 5e59bc428..9e582305d 100644
--- a/src/config.zig
+++ b/src/config.zig
@@ -314,6 +314,15 @@ pub const Config = struct {
     /// This does not work with GLFW builds.
     @"macos-option-as-alt": OptionAsAlt = .false,
 
+    /// If true (default), then the Ghostty GTK application will run in
+    /// single-instance mode: each new `ghostty` process launched will result
+    /// in a new window, if there is already a running process.
+    ///
+    /// If false, each new ghostty process will launch a separate application.
+    ///
+    /// Debug builds of Ghostty have a separate single-instance ID.
+    @"gtk-single-instance": bool = true,
+
     /// This is set by the CLI parser for deinit.
     _arena: ?ArenaAllocator = null,