Thorsten Ball cac5b00d94 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]
```
2023-09-05 13:59:07 +02:00
..
2022-12-31 08:53:11 -08:00
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