This removes `launched-from` entirely and moves our `gtk-single-instance`
detection logic to assume true unless we detect CLI instead of assume
false unless we detect desktop/dbus/systemd.
The "assume true" scenario for single instance is desirable because
detecting a CLI instance is much more reliable.
Removing `launched-from` fixes an issue where we had a
difficult-to-understand relationship between `launched-from`,
`gtk-single-instance`, and `initial-window`. Now, only
`gtk-single-instance` has some hueristic logic. And `initial-window`
ALWAYS sends a GTK activation signal regardless of single instance or
not.
As a result, we need to be explicit in our systemd, dbus, desktop files
about what we want Ghostty to do, but everything works as you'd mostly
expect.
Now, if you put plain old `ghostty` in your terminal, you get a new
Ghostty instance. If you put it anywhere else, you get a GTK single
instance activation call (either creates a first instance or opens a new
window in the existing instance). Works for launchers and so on.
587f47a587