I made a few minor changes. They include:

1) I edited the firepower icons and mode 5 different ones, each one representing a different level of firepower. Now, when the player has a specific fireppower level, the respective icon is displayed.

2) I added another case to the switch statement for the key pressed and key released. Now, the game will also keep track of the player input during the LEVEL_NEXT state so current input variables are preserved between level transitions.
I forgot to add the powerup and attribute classes required for the players thrust. I also cleaned up the PlayerEntity class by removing old commented code.
I merged the development branch with the master branch. I made a change to the way the firepower is displayed and I added case statements to the key release and key pressed handlers so user input is preserved and updated during the LEVEL_NEXT state.

Merge branch 'development'
I cleaned up some unused imports.