Some more changes.
I made some more changes to the main file in converting it to an Applet/JApplet
I changed references from Entoty to Entity2D
I added logic for a new smaller and more powerful UFO.

I cleaned up some of the code to display the player ship images from the Asteroid class and moved it into the PlayerEntity class.
A couple of things:

I found a bug with the scoring mechanism. Here I was adding the point values of "killed" enemies in the user defined collision method. Howewver, given some of the advanced enemies that are not killed with a simple call to their respective kill methods (e.g., the strong and shorty UFOs), their point values were prematurely added to the player's score each time the UFO was hit by a player shot regardless if the UFO was killed or not.

To correct this problem, I moved the scoring out of the user defined collision methed and added it to the respective kill methods of the enemy entities. Now, when an enemy entity truely dies, the point value of the enemy is added to the player's score.

Note that this further follows good object oriented design practices since each enemy knows how it should die (e.g., add its point value to the player's score and then clear its alive and visisble flags).

I also forgot to add the UFOShorty class to the repo in the last commit.
I fixed a bug with the powerups. When a collision was being processed for a powerup, the point value was being counted twice (e.g., a 250 point power up was being count as a 500 point powerup). I fixed the login in the user defined collision method that processes a collision between two entities.
I started to add comments to add the new logic for increased health and shields pwoerups.
I merged the other branches with the development branch

Merge branch 'testingrefactoredentityclasses' into development