Split SpaceTrader into FXML files and Controllers.

Removed all graphics code from SpaceTrader.java
Created WelcomeScreen.fxml and WelcomeScreenController. SpaceTrader now sets the screen to the WelcomeScreen.fxml and WelcomeScreenController sets the scene to the CharacterDialog.fxml when the start button is clicked.
Combined the difficulty config screen with the skill config screen.
Added toolips for each skill category in the skill config screen. Added picture files.