Add a field to modify unresponsiveness detection timeout

### Problem

Unresponsiveness detector is bugged, so that accidentally clicking to force quit a process which already stopped crashes the whole session (https://github.com/elementary/gala/issues/2453).

I also think a configurable timeout is generally useful, for already-existing apps which are known to have long computations.

### Proposal

Create a field, interface for `gsettings set org.gnome.mutter check-alive-timeout ` (in the command, 0 stands for "do not even check if app responds").

### Prior Art (Optional)

_No response_