PDAs open their messenger when replying to messages through chat (#68355)

* Clicking Reply in PDA messages now opens your PDA messenger app.

* Additionally does a ton of PDA code improvement.

code/modules/modular_computers/file_system/computer_file.dm

@@ -3,6 +3,7 @@
 	var/filetype = "XXX" // File full names are [filename].[filetype] so like NewFile.XXX in this case
 	var/size = 1 // File size in GQ. Integers only!
 	var/obj/item/computer_hardware/hard_drive/holder // Holder that contains this file.
+	var/obj/item/modular_computer/computer
 	var/unsendable = FALSE // Whether the file may be sent to someone via NTNet transfer or other means.
 	var/undeletable = FALSE // Whether the file may be deleted. Setting to TRUE prevents deletion/renaming/etc.
 	var/uid // UID of this file
@@ -18,9 +19,10 @@
 
 	holder.remove_file(src)
 	// holder.holder is the computer that has drive installed. If we are Destroy()ing program that's currently running kill it.
-	if(holder.holder && holder.holder.active_program == src)
-		holder.holder.kill_program(forced = TRUE)
+	if(computer && computer.active_program == src)
+		computer.kill_program(forced = TRUE)
 	holder = null
+	computer = null
 	return ..()
 
 // Returns independent copy of this file.