[GUI update] Accumulator should indicate charge level on GUI

[y.petremann]

Based on the mockups on the GFX wiki, I've seen that the Heat exchanger have a bar indicating his temperature. On this idea I think that Accumulators should indicate their charge level.

[picklock]

+1 from me. This is a good idea,

[gGeorg]

Also Nuclear reactors should show bar of cell consuming.
reactor is the only burner/consumer which doesnt show status of the fuel.