Shift+F3 debug menu (#18065)

* F3 debug menu

* Some tweaks

* So that escalated

* Update code/controllers/subsystem.dm

* Update code/controllers/subsystem/debugview.dm

* Charlie tweaks

code/modules/client/client_defines.dm

@@ -65,6 +65,9 @@
 	//datum that controls the displaying and hiding of tooltips
 	var/datum/tooltip/tooltips
 
+	// Overlay for showing debug info
+	var/obj/screen/debugtextholder/debug_text_overlay
+
 	/// Persistent storage for the flavour text of examined atoms.
 	var/list/description_holders = list()
 

code/modules/client/client_procs.dm

@@ -415,6 +415,7 @@
 
 /client/Destroy()
 	announce_leave() // Do not put this below
+	SSdebugview.stop_processing(src)
 	if(holder)
 		holder.owner = null
 		GLOB.admins -= src