Groundwar: attack every building behavior problems

```
In a recent issue (Issue #514) it was requested that AI should attack not just
vehicles and defenses, but to "spend" time on regular buildings so the defender
can reach them.

Since the player would probably not target all buildings in his path if he/she
aims for capturing a mostly intact planet, this new behavior seems to be
problematic: usually, the player needs to wait until a few remaining units make
through the city to the last defensive building where they get destroyed.
Demolishing remaining defenses is an option which yields a battle over
situation, but only in clearly lost battles.

If the power dependent defenses is introduced, this may lead to further
complication: a single power plant destroyed can cripple the defenses but if
the AI destroys regular energy-drain sources, it just makes things harder for
themselves.

The resolution could be the following behavior:

- Damage power plants up to 51% to make them disabled if no active fire brigade
is present, otherwise, destroy power plants until a significant power shortage
is caused.
- Damage fire brigade (51%) if present
- Attack vehicles
- Attack defenses

```

Original issue reported on code.google.com by `akarn...@gmail.com` on 21 Jun 2012 at 12:00