Add status bar, change spaceship speed units

* Status bar shows speed for spaceships for now.
* Integers are used for speed/acceleration/movement calculation to avoid
floating point rounding issues.

Commit references #9.
Inertia is spaceship movements.

* Spaceship has N independent movement counters, where N is number of
pointers available.
* Change speed measuring scale to 1..1000. Set maximum speed to 500.

Commit references #9.
Merge branch 'inertia' into develop

Commit closes #9.