Add Invader class

It does not know about its surroundings and it does not move on its own.
Something external should move it with a single Timer for the whole game.

If the Invader had a Timer there would be a lot of separate Timer instances in the game.
Add Space class to control the invaders

It is responsible for creating new invaders on random positions and moving them down.

Still work in progress: the new invaders are not always visible.
Use a panel container inside the frame

Extract it to a new class called SpacePanel
Improve the Invader a bit and add logging

The default step is now the Invader height.
Merge pull request #9 from zonama-sekot/invader

Invaders & Space