Hotplate overhaul (#17586)

* Hotplate overhaul

- Beakers are now placed on top of the hotplate instead of itemslot.
- Hotplate LED now only lights up if it's heating something.
- More comments/xmldoc.
- Some other minor tweaks.

* Actually remove the beaker slot

Content.Shared/Chemistry/SharedSolutionHeater.cs

@@ -0,0 +1,9 @@
+using Robust.Shared.Serialization;
+
+namespace Content.Shared.Chemistry;
+
+[Serializable, NetSerializable]
+public enum SolutionHeaterVisuals
+{
+    IsOn,
+}