Recharger QoL Tweaks (#3826)

changes:

Rechargers now briefly show a progress bar showing the current charge level of the device currently charging.
Examining a recharger will now specify what is in the charger.
Atoms can now mark themselves as requiring an icon update on initialize instead of overriding Initialize() just to call update_icon().
Charger code is now slightly more modular, using proc/get_cell() to get a ref to the power cell instead of hard-coded if-else chains for each supported type.

code/modules/modular_computers/computers/modular_computer/variables.dm

@@ -59,3 +59,5 @@
 	var/obj/item/weapon/computer_hardware/tesla_link/tesla_link						// Tesla Link, Allows remote charging from nearest APC.
 
 	var/listener/listener	//Listener needed for things
+
+	charge_failure_message = " does not have a battery installed."