point out missing energy more clearly

[nczempin]

• when you have no energy, and none is being produced
• when your towers would like to spend more energy than is currently being produced

The point at which you hit 0 energy can be accompanied by a loud noise, some message and flashing of the energy display. Thereafter the energy-using towers could be "greyed out" somehow.

When you are producing energy, but less than you would like, there should be clearly defined rules as to which towers get priority. At the moment it is simply arbitrarily decided by which tower got placed first.

[nczempin]

TODO: create separate issues for the detailed effects.

[nczempin]

low energy shooting rules: #153

[nczempin]

for now, let's just draw something red around the energy display when the energy is 0.

[nczempin]

implemented in a32a6a5fae3054280194ab55c805b71d53dc2b0f