gtk-ng: deprecate detection of launch source

Detecting the launch source frequently failed because various launchers
fail to sanitize the environment variables that Ghostty used to
detect the launch source. For example, if your desktop environment was
launched by `systemd`, but your desktop environment did not sanitize the
`INVOCATION_ID` or the `JOURNAL_STREAM` environment variables, Ghostty
would assume that it had been launched by `systemd` and behave as such.

This led to complaints about Ghostty not creating new windows when users
expected that it would.

To remedy this, Ghostty no longer does any detection of the launch
source. If your launch source is something other than the CLI, it must
be explicitly speciflied on the CLI. All of Ghostty's default desktop
and service files do this. Users or packagers that create custom desktop
or service files will need to take this into account.

On GTK, the `desktop` setting for `gtk-single-instance` is replaced with
`detect`. `detect` behaves as `gtk-single-instance=true` if one of the
following conditions is true:

1. If no CLI arguments have been set.
2. If `--launched-from` has been set to `desktop`, `dbus`, or `systemd`.

Otherwise `detect` behaves as `gtk-single-instance=false`.

src/apprt/embedded.zig

@@ -909,7 +909,7 @@ pub const Surface = struct {
             // our translation settings for Ghostty. If we aren't from
             // the desktop then we didn't set our LANGUAGE var so we
             // don't need to remove it.
-            switch (self.app.config.@"launched-from".?) {
+            switch (self.app.config.@"launched-from") {
                 .desktop => env.remove("LANGUAGE"),
                 .dbus, .systemd, .cli => {},
             }