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.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/1080

src/meta/prefs.h

@@ -105,6 +105,7 @@ typedef enum
   META_PREF_CENTER_NEW_WINDOWS,
   META_PREF_DRAG_THRESHOLD,
   META_PREF_LOCATE_POINTER,
+  META_PREF_CHECK_ALIVE_TIMEOUT,
 } MetaPreference;
 
 typedef void (* MetaPrefsChangedFunc) (MetaPreference pref,
@@ -481,4 +482,7 @@ gboolean           meta_prefs_bell_is_audible      (void);
 META_EXPORT
 GDesktopVisualBellType meta_prefs_get_visual_bell_type (void);
 
+META_EXPORT
+unsigned int meta_prefs_get_check_alive_timeout (void);
+
 #endif