display: Make check-alive timeout configureable

The check-alive feature is there for the user to be able to terminate frozen applications more easily. However, sometimes applications are implemented in a way where they fail to be reply to ping requests in a timely manner, resulting in that, to the compositor, they are indistinguishable from clients that have frozen indefinitely.

When using an application that has these issues, the GUI showed in response to the failure to respond to ping requests can become annoying, as it disrupts the visual presentation of the application.

To allow users to work-around these issues, add a setting allowing them to configure the timeout waited until an application is considered frozen, or disabling the check completely.