- open GUI when an electric-energy-interface entity is selected
- electric-energy-interface entity can provide circuit signal about the amount of its stored energy
- just like the behaviors of accumulator
Since the example shows that electric-energy-interface can be used as an accumulator, so I guess these requests are fair?

Because I was making configurable energy output and input for my mod, and I wanted to use player.opened to open the configuration GUI. But then I was surprised that electric-energy-interface doesn't have GUI, so it doesn't work on player.opened.
By the way, is it a bug that when usage_priority = "primary-output"...
- if I set energy_usage to "0W" with energy_source.input_flow_limit higher than 0W, animation is not played
- if I set both energy_usage and energy_source.input_flow_limit to higher than 0W and there is no other energy source in the network, animation is played but the Low Power warning is shown when power saturation = power production in game (e.g. there are accumulators absorbing energy) even though I set its entity.energy to be sky high in each tick (so I guess it cannot be used as a generator?)
- if I set both energy_usage and energy_source.input_flow_limit to "0W", animation is not played and Low Power warning is shown when saturation = production