Added the first version of Upgrading for the Spaceships with an overlay in the PlaneChooserScreen, still needs refinement and new values for ships are not yet used in the game.
Merge branch 'master' of https://github.com/FAU-Inf2/fly-gdx

Conflicts:
	android/assets/spaceships/json/Spaceship2.json
	core/src/de/fau/cs/mad/fly/features/game/EndlessLevelGenerator.java
	core/src/de/fau/cs/mad/fly/profile/PlaneManager.java
adjusted the new planes to the upgrades and added a new background to the PlaneChooserScreen