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.

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`.

Fixes #2473

This commit changes `ghostty_surface_ime_point` to return a full rect
with the width/height calculated for the preedit.

The `firstRect` function, which calls `ghostty_surface_ime_point` was
previously setting the width/height to zero. macOS didn't like this. We
then changed it to just hardcode it to width/height of one cell. This
worked but made it so the IME cursor didn't follow the preedit.

This was a noop in all of our apprts and I think is a holdover from the
glfw days.

This is a tiny addon from the recent gtk-ng work. We've moved redraw
requests into the apprt action system (the `render` action). I waited
until I had my macOS machine to verify that this fix could work. We can
now remove this completely.

We can probably remove the redraw inspector API too at some point but
I'm not there yet with the GTK backend so I'll just wait on it.