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]
```
This is part of #319 by fixing it for GTK and introducing the
configuration option.
This adds `window-decoration = false` as a possible configuration
option. If set to `false`, then no window decorations are used.
macOS translates inputs such as shift+tab into the control character tab
(ascii 0x09). Linux/GTK does not translate character inputs except to
printable characters. We don't want control character translations
because these are all handled manually by our key encoder (i.e.
translating ctrl+c to 0x03).
To determine the logical key that was pressed, we previously just
trusted that the translated text would have the right value. But if
modifiers are pressed, the text may not translate.
For example on macOS, Ctrl+C does not produce any text. As a result, we
would fall back to the physical key. On layouts like Dvorak, the
physical key for "C" is "I". This means "Ctrl+C" sequences weren't
working.
Instead, if there is no text or the text doesn't map to a key, we
translate again using no modifiers to try to get the raw text of the
input and then base the key on that.
Fontconfig in particular appears unsafe to initialize multiple times.
Font discovery is a singleton object in an application and only ever
accessed from the main thread so we can work around this by only
initializing and caching the font discovery mechanism exactly once on
the app singleton.
